In a Hex5 without a heater, you are SEVERELY limited. I wouldn't suggest having a betta or pea puffer in there unless you can GUARANTEE that the temperature in the tank will stay above 76F. I wouldn't put danios/glofish in such a small tank. They are insanely active and require 20gal minimum. Cories prefer being in groups of 4 or more and even the pygmies are too active for a 5gal (especially one with as small of a footprint as a Hex5...trust me, I know...I have one).
Here's an idea...why don't you spring for a $14 Tetra heater at Walmart? That'll open up some possibilities for the fish you can keep in the tank. You could always do the tank as an inverts-only tank with red cherry shrimp, maybe a dwarf orange crayfish and a nerite snail. Get a nice tall, branchy piece of driftwood, plant a whole bunch of low light plants (anacharis, frill, cabomba, java fern, java moss) and viola! Colorful, beautiful inverts tank.