i'd go on ahead and remove everything. the easiest and fastest way to reduce the ammonia in that 10g is to empty it and start fresh. all your bacteria is living on a surface somewhere; your water is just full of poison.
the betta will deffo eat any fry in that 2.5g; since that's also the only safe place for the neons, i'd put them in there and put the betta in a bucket. i'm hesitant to add the platys to the 29g for fear of the butterfly eating them, but then i'm used to the dwarf variety.
you've obviously already got plenty of ammonia in that 10g! it's going to eventually cycle whether you add more ammonia sources or not. i suggest a heavy dillution of the ammonia followed by a slow readdition of fish. basically, a partial fishless cycle followed by a fishy cycle. it'll be a bit tough on the neons, but it can be done.
have you got any plants? i've got an excess at the moment; if you want or need any, drop me a pm and i'll try mailing you some. live plants are magnificent for managing wastes in a tank.