The first spawn will probably have to be in the summer so that they are both mature! I believe it is doable. It's simple selective breeding. Just guessing here, but I believe it will work something like this-
1)MicroxTeeny. Two runts breeding together, average amount of runts expected.
2)Next few generations- number of runts goes up.
3)Almost all runts (f5?)
4)Decreased adult size
5)Smallest selected until we get small, stable bettas
So yea, it will take a while. And somewhere along the way we will have to outcross to another unrelated runt.
Btw bb123 are you spawning over spring break?