I hate how little control you really have over your faction, it's doctrine, and the general direction of it's fleets, I ask for a particular weapon to be used, and no ship will use it, even if it's the only weapon prioritized! I'd like to design ships from a template and then have those. I'd also like to be able to deprioritize ships so they DON'T show up in my fleets!
Tell me, is this even possible?
Yes, to a certain degree. Setting up fleet doctrine so that a ship is likely to appear is the first step. Smaller ships, and mainly combat ships if you want a fleet of Scarabs.
After that you prioritize Scarabs. For the weapon you want them to use, you prioritize it, for this example I'll use tactical lasers.
Everything so far is what you have already done.
For loadouts your fleets draw from the vanilla loadouts, and your custom loadouts. So, you go into the refit screen, refit your Scarab with five tactical lasers, ITU, advanced optics, and advanced gyros, save your loadout with a new name for example lightshow, and remove all other loadouts.
From now on your fleet will be mostly Scarabs with the lightshow loadout as that is the only loadout that makes use of tactical lasers.
Other ships and loadouts will still show up, but far less frequently.
All spawned fleets have various roles, while you can influence those roles with fleet doctrine, they can't be flat out removed. So always prioritize one cargo ship, one carrier, and one ship of each size.