Sexing Blue Rams is fairly easy. The second ray of their dorsal fin is extended on the males, and when they grow older the 4th ray will also grow to be extended.
I used to have a few of these. Unfortunately, I had one dominant male who would harass not just the other rams but all the other fish in the tank, including my 2 adult angels who are about 10 times the size of him. So, I had to take him out of that set-up. He did actually harass a female to death once.
I too bought these fish on the understanding that they were peaceful little fish. Mainly they are, just some males can be dominant like this.
If you want to keep more than one, it is best to have at least 2, maybe 3 females to each male. This cuts down on the aggressive behaviour (or at least spreads it out so not just one female kops it)
Hope this helps.