Scarabs can be hella fun. Keep them away from template weapons though otherwise you'll be losing bases left and right. They work great against fortifications and pretty much anything with armor that you can get close enough to. They are beasts so they get 12'' movement which helps them with that.
Essentially how it works is against stuff with armor values (vehicles/buildings) for EVERY hit you score roll a dice; on a 4+ the vehicle or building loses 1
AV point on all sides for the rest of the game.
Ex. I assaulted my friends Land Raider (AV14 all sides) in a game with 5 scarab swarms; I get 25 attacks (4 + 1 for the charge) hitting on 3's since it moved. I ended up rolling about 16 hits on it. I then rolled 16 dice and 8 of those were 4's or better. It's
AV is now 6 on all facings (AV14 - 8). Easy pickings now. Same thing goes with buildings and fortifications; except you automatically hit on those
Against infantry I think you just have to score an unsaved would on the model and it loses its armor save for the rest of the game.
As far as your list goes I usually run my wraiths in groups of 3 - 4; I havent ran a monolith for awhile, I generally use the Anihillation barges which you can bring two in exchange for it with 20 points left over. So if you drop the partical caster and maybe some warriors or something you can squeeze in another 1 or two wraiths as well.