No, that would be pretty crowded. I've always felt that Neon Tetras shouldn't be kept in aquariums smaller then 10-gallons. The reason is that they do best in schools (of at least six) which would be over-stocking any small "desktop" tank.
Platies might be ok but stick to two or three of the same gender and don't get anything else.
Zebra Danios are far too active for desktop aquariums. They need a lot of room to swim.
A 5-gallon really can't hold more then two or three small fish and the same goes for a 6-gallon (they are very similar in size). Exceptions would be "Dwarf Lifebearers" aka Heterandria formosa
, Cherry Barbs
, and Endler's Livebearers. Small groups of four or five fish like that should be ok if the tank is properly cycled and well maintained. Keep up on water changes and maybe add some live plants.
The problem with such a small tank is keeping everything stable. One small mistake that wouldn't matter in a large tank could cause the cycle to crash in a small one. Test the water on a regular basis and be very careful not to overstock.
Some better fish choices other then the ones I listed would be Fancy Guppies (two or three), most Killifish (a pair), a single Dwarf Puffer, a single Betta, White Cloud Minnows (three or four, perhaps?), a single Paradise fish, or a single Pygmy Gourami. Some bottom-dwellers who do well in smaller environments are the pygmy/dwarf cory cats
, oto cats (these guys can be more sensitive and should be acclimated slowly and kept in a group of at least three), and ghost or cherry shrimp. Freshwater Nerite Snails and Freshwater Clams can work too.
As you might've guessed by now it's very difficult to have a 5-gallon "community". The closest you might get to that would be keeping a Pygmy Gourami, some Dwarf Livebearers, a couple Nerite snails, and pygmy cory cats. Shrimp might get eaten but can fit into a lot of set-ups.
Any fish you get should be small when FULL GROWN...under two inches or so. They shouldn't be fish that require large schools, a lot of swimming space, or pristine water conditions (which will be hard to duplicate in a small "community"). Avoid fish that reproduce quickly like guppies unless you stick to one gender or have a plan for the eventual babies.
I've found that it's best to do "species" tanks instead and stick to one small species of fish.
No matter what you choose start cycling the tank now so it's safe for fish.