You can still spawn cultists, Flux Stabilizer just makes them show up in adjacent rooms instead of the altar room. You can do this even though the adjacent rooms don't have altars. The "if able" clause is there, I believe, to protect against the possibility of there being no adjacent rooms, which could happen if they ever make a scenario with a room accessible only by Ladder.
There is no limit to how many monsters can be in the same space, even if it's hard to make them physically fit. (Shoggoths can fit through doors because they are amoeba-like with no rigid form, though this obviously doesn't translate well to plastic minis.)
thx 4 quick answers m8!