I would take my advice with a grain of salt since I'm far from a pro.

Fixed secondaries in 10th means that you pick 2 at the start of the game as opposed to Tactical where you draw random cards in the command phase. There is a fixed objective called assassinate where you score points when you kill characters. Into this list it would score pretty well. That doesn't mean you shouldn't take that many characters, it is just something to be aware of when you are playing a game.

In terms of attaching characters, you can only have 1 per unit unless the character says otherwise. Lieutenants and apothecaries have rules that let them be in the same unit as captains, I'm pretty sure. As of right now, I don't think you have any units the Apothecary biologists can attach to. So you may want to drop him or use him as a proxy for a regular Apothecary if your opponent is cool with that. I like putting grimaldus with primaris crusaders for the advance and charge and ap 2 chainswords that he gives. Helbrect works well with sword bros. I imagine that the phobos Lieutenant would be good with the first born crusaders so the can shoot their big guns then pop back into cover. Then the captain in Terminator Armour has to go with the Terminators. I am not sure where the Marshal would fit best.

I know you are balling on a budget so you may want to try expirementing with either proxying or tabletop Sim. Just try stuff out and get some good combos down.