Well...you can kinda force RNG.Snail boss not yet, small snail or slug is already implemented, you can encounter them on map 3 and 4 again RNG based so if you are unlucky, well not going to meet them then.
If I had to guess how this game save's work, it keeps the type of room in saves, and then when you enter it generate the monster according to the type of room.
So if you know which type of room spawn what monster, you can just quit back to menu and reload to force RNG...still RNG, and if you just don't get the right type of room, you are good to restart.