In a 55 I would recommend 1 male and 5 females if you want a nice group and no real aggro problems. Maybe go 2 electric Blue, 2 Gold and 2 normal. The girls have pink bellys normally, they're pretty easy to sex. Your best bet is also to introduce all at the same time but if you already have 2 you should still be ok if you put the 4 in together, might be a bit rough for a few days. Add extra hiding places at the same time as the new fish.
Won’t the females be aggressive with one another?
My ram experience is VERY anecdotal but my male is the guard dog during spawning. At least in a 10G where they were being quarantined with a small group of cardinals. He lost his mind until I got the cards out of their. If I was serious about spawning rams I would never have anything else in the tank unless it was a large one. As Kasshan mentioned I am not overly impressed with my GBRs parenting abilities so far.
I don’t really plan on breeding. Is it possible they already have if the male is staying in the back corner? The female moves around but he tends to stick to a 3” radius of one of the corners of the tank which to be fair has 2 large flat rocks.