A couple weeks ago I moved 2 guinea cocks and 1 guinea hen in with my 2 peacocks and 1 peahen. The guineas were a bit nervous at first and didn't like the peas, but they got used to them after a couple days. Everybody gets along fine now, the guinea hen is laying eggs (Just set some eggs from her yesterday) and the two guinea cocks are getting a bit more aggressive with each other, but they don't bother the peacocks. The peacocks have been chasing each other a bit, but they also don't seem to bother the guinea cocks.
The main thing I would say is to watch out for Blackhead, because I was told guineas are possible carriers and could give it to the peas. My peas are housed in a pen right next to my chicken run, so it didn't seem to be a problem when I introduced the guineas directly into their pen.