To be honest, I've never mixed sizes of Peppermints. I would guess that the smaller would be fine if they were mature. I do know that Peppermints will eat their own larvae. I'll try placing a small one amongst the larger colony tonight and let you know what happens.
The larval period is about 2 months, sometimes less, and they reach maturity about 3 months after that in my experience. I would guess that the available foods could have a large impact on that timeframe though. Certainly within 6 months post-larvae they would be breeding, probably much sooner though. The shrimp you get from the LFS are probably mature and ready to breed.