1. you may have problems if you have a male betta with guppys, esppecially if they have decent sized tails.
2. probably need a tank 600x300x300 with a filter and heater.
3. the guppys will bred like made if you give them the right conditions, bettas are much more complex and you would want to do alot of reading before giving this a go. (don't have a single female witha male) and neons are very hard to breed, not as hard as some fish but don't hold your breath. I've tried breed all three of these fish, lots of success with guppys, some with betta and non with neons.
This is just from what i know, and from my experiances, other options my differ.