I would say in a 10 gallon with 1 betta you would only want about 5-6 more fish, and since platies and cories are shoaling fish, it would have to be one or the other if you want a happy non fighting tank, also neons are a little too bully-able (IMO)if your betta decided he wanted his tank back to himself, so I would say your best bet are platies because they are quite hardy fish and can stand up for themselves, dont get all males, they will kill each other because there are no females to breed with, I would say 4female 2male
Proud mum of : Gandalf, Lily,(ohm bettas)
Pineapple (male betta)
Jade, Indigo (hm bettas)
a few platys, a few guppies,
a few shrimp,
and whatever else sneaked into my tank while I wasnt looking
If it moves. . .BITE IT!!!