Well I hate to be the bearer of yet other bad tidings but neons will have your bettas fins in shreds. They are known to be fin nippers and he will not consider them a threat and will just let them approach him and do it. The funny thing about the combination of these guys is I have seen YOU TUBE shots of the neons simply pecking away as the betta just kind of floats in the water unaware that anything is going on. It is horrifying to watch. Then soon the bettas fins are infected with fin rot and you have a problem.

There is a tetra that is colorful and really quite beautiful that will not do this to my knowledge and has not ever caused a problem and you may want to consider substituting for the neon and that is the Cardinal tetra. This is the only one that I would risk putting in with a betta as they are the only one I know of that has not ever been a nipper to my knowledge and after the number of years I have been about them I am sure I would have had someone tell me.

But back to your question, when you are going to have tankmates with a betta, it is always really best to put all the other tanks inhabitants in the tank and have them all settled in and happy before the betta goes in. He is going to be the star of the show we know but he needs to take the territory that is left AFTER the others have established theirs rather than allowing him to think that the whole tank is HIS and the others are intruding. That is a recipe for disaster and usually the start of a fight. This of course is only if you are not setting up an aggressive tank which I would never suggest that a betta be added into. And with the fish you have mentioned I do not see that happening. But I mention it for the benefit of any others who may someday consider it. Bettas fins keep them from swift motion and do not allow them to protect themselves in an aggressive tank against fish that are able to get around more nimbly.

Rose