unless you have a filter or some kind of aeration in the 5G you wouldnt' be able to have platies, since they need oxygenated water, and don't breathe off the surface of the water like bettas.
ADFs, or a snail, or some ghost shrimps might be good tank mates. If you choose to go with otos (i'm pretty sure they need oxygenated water too), make sure they're in a group of at least 3
You could try some small peaceful bottom dwellers like cories or ottos. Ottos can be useful in keeping algae down and cories are just sweet but if you get cories make sure you have fine substrate for them.