Mine do that too. It's establishing dominance. My alpha female in the tank will even scare off males that are not her mate. They are very fun to watch in a community. Also their colors will come out allot more when they pair off and spawn.
I been watching one of my rams and she has been chasing around my tetras I'm pretty sure she killed two of them because everything else was fine. I had to pull her out of the tank and put her in my 3g pictope until I figure out what to do.
I'd have to go both blue, however the second one may be younger than the first, what's making me think that is that there is a lack of coloring that the first female has, the nipping may be them establishing dominance like someone else said
I wish you where local to me I could use more females. And I've got wild caught males. I would trade. Talk to your lfs see if they will order a big group and then you can pick and choose. If not matt13 on this forum or aquabid. I would check matt first though. From what I've seen his rams are great.