spotted, florida, and tropical gars should be able to stay in a 75 for 6-8 months, and possibly a bit longer, provided the tank is 18" wide, and they're good hardy gars to keep. To keep them for life, you really need a tank that's at least 2.5ft wide, or better yet, 3ft wide.
anyway, these three types of gars can generally make good bichir tankmates, provided they are of compatible size and you don't have a psycho individual (most aren't, so don't worry

). Gars aren't strictly mid/top level though, when they get comfortable with the surrounding, they'll sit on the substrate just like the bichirs (pretending to be bichirs is what i always say

)
ps. i'm not sure what the realistic max size for spotted/florida are in captivity, but for tropical gars with their wider mouth, you really only wanna keep it with medium to large species of bichirs eventually, or you risk the gar eating the tiny (relatively speaking) bichir.