While it looks like several of the formations that include Scarab Occult Terminators will be very expensive points-wise (and so difficult to build into a normal CSM army), it looks like you will be able to take this unit as part of a normal Thousand Sons CAD, which enables them to benefit from the Thousand Sons special rules (basically, Chapter Tactics for Thousand Sons), in particular the Blessings of Tzeentch rule, which increases their Invul Save by +1 if they have a blessing cast on them and have VOTLW rule (which they get for free, also thanks to Thousand Sons special rules).
This means that, with a blessing cast on them, Scarab Occult Terminators become 2+/3++ Terminators that also have Power Swords, TLed AP3 Bolters, and the option to add a Hellfyre Missile Rack (24″ S8 AP3 Heavy 2) for every 5 models and either a Heavy Warp Flamer (S5 AP3 Template) or a Soulreaper Cannon (24″ S5 AP3 Heavy4 Rending) for every 5 models (so a 10 man squad could have a total of 4 Heavy Weapons). In essence, you are getting the best of both Tactical Terminators (with lots more AP3 shooting) and Assault Terminators (minus AP2 close combat weapons) in a single package. Finally, the Scarab Occult Terminators come with a PML(2) Sorcerer as their squad leader “base,” which gives them lots of options for how to get those blessings off.
Whether using their own “organic” Sorcerer or an attached Thousand Sons IC (like Ahriman), I think these are some of the best Blessing Psychic Powers to use in conjunction with them:
 Warp Fate (Sinistrum) — THIS is the single power that you want to roll above everything else. First of all, it is a Blessing, so if cast successfully it brings the Scarab Occult Terminators up to a 3++ save. Secondly, while it is WC2 (Thousand Sons bonuses against Perils of the Warp will help with this), it allows the unit to re-rolls ALL of its saves… Armor, Cover, and Invul saves included. That means that the Scarab Occult Terminators will have a re-rollable 2+ save against all non-AP2/1 attacks and a re-rollable 3++ against any of the attacks that ignore their Terminator armor save. This makes them almost invincible against most types of attacks and even dedicated “terminator killers,” like Grav Cannons/Amps, would take an average of 13 shots to kill a single Scarab Occult Terminator.
 Earthly Anathema (Geomortis) — Granting Ignore Cover (and no LOS restrictions) to a unit with tons of AP3 shooting potential makes the Scarab Occult Terminators very strong against common units like Space Marine Bikers, massed MEQ units, Eldar Jetbikes, Necron Phalanxes, and even light infantry armies, like Genestealer Cult and Astra Militarum, that tend to cling to cover a lot for their saves. This is a great power for any shooty unit and Scarab Occult Terminators in particular can benefit from it. Don’t forget, as a Blessing, this also buffs the unit’s save up to a 3++.
 Ghost Storm (Ectomancy) — Giving a unit a “free” 18″ move in the Psychic Phase is good for almost any army, but works especially well on Scarab Occult Terminators in a Thousand Sons force, which will likely have mobility as one of its key handicaps. Given that Scarab Occult Terminators’ firepower is pretty much all 24″ or shorter in range, this is a very solid Psychic Power that helps them maximize their combat power and durability, while also mitigating their normally poor mobility (and gives them a 3++ for “free” as well).
 Invisibility (Telepathy) — Just like with Warp Fate, this is one of the best powers to make a unit of Scarab Occult Terminators almost invincible to normal attacks, especially when combined with a 2+/3++ profile. If your opponent is not savvy, he might be tempted to pour all kinds of attacks onto an Invisible Scarab Occult Terminator squad and end up wasting all or most of it, thus leaving the rest of the army free to maneuver with minimal interference (just for reference, it would take an average of 72 Bolter shots or 21 Plasma shots to kill a single Scarab Occult Terminator with Invisibility cast on it). Bottom line, Invisibility makes Scarab Occult Terminators an excellent “anvil” type unit that can draw attention away from the rest of your army and still survive to contribute in its own right.
 Endurance (Biomancy) — While Scarab Occult Terminators are already Relentless and only have 1W (except for their Sorcerer squad leader, who has 2W, according to the rumors), making Eternal Warrior no real advantage, giving the who squad FNP(4+) is major buff to its survivability against anything S7 or below. Put another way, when combined with 2+/3++ saves, Endurance in essence doubles the survivability of this unit, in turn forcing your opponent to devote twice as many attacks (on average) to deal with it.
 Prescience (Divination) — Everyone knows how good this already… combined with strong shooting and solid close combat abilities, plus the ability to Deep Strike in close to the enemy and the 2+/3++ saves from Thousand Sons special rules/a successful casting of this Blessing, and you have a unit that is very good in both shooting and close combat (well, very good against anything that does not have a 2+ save, so most opponents you will face). This probably makes for one of the most versatile combinations for a Scarab Occult Terminator squad, allowing them to be strong at a whole variety of mission sets.
 Fleshmetal Hide (Heretech) — Improving Scarab Occult Terminators to T5 is a solid buff against most small arms attacks by itself, but where this can REALLY shine is if you combine it with the new formation that makes Scarab Occult Terminators T5 base if they are within a certain distance of their formation leader… put these two together and then you have T6 2+/3++ Terminators, putting the entire squad on par with Dreadknights (with Sanctuary cast) or Riptides (with Nova Reactor upping its save to a 3++) in terms of durability.
Based on all this, the ideal layout for a “mini-deathstar” squad of Scarab Occult Terminators, IMHO, would be something like this:
-Exalted Sorcerer with PML(3), Familiar, and Astral Grimoire Relic
-10 Scarab Occult Terminators with 2 Hellfyre Missile Racks, 2 Soulreaper Cannons, and PML (2) Sorceror as squad leader
Starting with the Exalted Sorcerer, I would roll hard on the Sinistrum Table, trying to get Warp Fate, as I think that is one of the single best powers for a Scarab Occult Terminators. If you succeed in rolling that up, then Squad Leader Sorcerer could roll on Telepathy to try and get Invisibility and, if not that, at least guaranteed Psychic Shriek for some AP2/anti-MC abilities. Taken together with the Astral Grimoire Relic, the whole squad becomes Jump Infantry and can either move 12″ a turn or, using its “Jump Packs” in the Assault Phase to re-roll charge distances and get Hammer of Wrath attacks.
Thus, if you get off Warp Fate, you have a squad that can move 12″ a turn, has 2+/3++ saves (both of which are re-rollable), and then has 8 x S4 AP3 Twin-linked shots, 8 x S5 AP3 Rending shots, and 4 x S8 AP3 shots at 24″ to deal with enemy units (and Psychic Shriek for extra damage where required). This is of course would make for quite an expensive unit and would be dependent on getting at least one good Psychic Power from the list described above (again, Warp Fate I think is the best option), but would also make for a highly flexible, mobile, durable, and dangerous unit that would be effective against almost any opponent.