To start, your guppys and mollys will interbreed. Platys, swords same thing. Least Killies, they are acutally livebeaers, would work out well and with out the above mentioned problem. Marble Crayfish, you only need one, would also reproduce and help clean up.
As for the livebearing fish, I highly recommend seperate tanks for different varieties. If you want a smoken livebearer community tank, say a 20L, then guppys, mollys, paltys and Endlers. I keep my Endlers and Least Killies together. Just a warning, Endlers and Least Killies don't eat their young. Two pairs of each and you will have thirty fish in four months. I have known people who raise Endlers and stock their show tank with all Endler males. It really looks unbelievable.