They just need time to settle.
Believe me---you'll know if things are going sour pretty quickly, because there'll be a hell of a lot more than just 'nipping' at one another. In fact, one fish (the loser, obviously, lol) will end up scrunched up into one of the top corners of the tank; probably behind the heater, if the sh_it does end up hitting the fan.
But, in all likelihood, I'm guessing they'll sort things out amongst themselves and get to breeding as soon as they're ready. They may need to grow some, but many folks have had em' breed at a fairly small size, as mentioned.
Just give em' more time and keep a close watch on them.
Let us know if anything else of concern pops up though!
BV