my bettas have always lived with guppies & mollies without problem--i think the male guppies become a problem if there isn't enough space (like in a 5 or less
gal tank), or there aren't any hiding places (caves, rock formations, etc.). however, i have never had any guppy/betta violence in either a 10
gal or 20
gal tank. of course, i also always provide a mirror for my betta's (ahem) enjoyment. ; ) bettas and livebearers tend to thrive at similar (higher) temps & pH, while neons need a _lot_ lower pH to survive (or at least, that's always been my experience). otos also do well with bettas....i have 4. they keep the glass clean, the algae down, and don't try to eat his food (his only real issue with his tankmates,
LOL). he pretty much ignores them, unless he's feeling curious--then sidles up next to them & watches them till they freak out & go to a different area of the tank. anyway, lots of nice, bright, vibrant colors with livebearers---mollies & swords, if you don't want to try guppies---and i've never had any problems with fry being eaten by the betta, either (
FWIW). i use aquarium salt & "Blackwater Extract" (adds vitamins, etc. to water), and all the fish thrive (not just the mollies & guppies), so you shouldn't have any problems there, either. never kept corys or gouramis, so i can't really comment on that, tho. just make sure to stay away from nippers (i also lust after puffers. sigh.
LFS had the cutest dwarf puffers for $1.99 each. almost cried.
LOL) and make sure they are in the same pH and temp range (and will all fit after reaching full growth), and you'll do fine.