I have a pair of blue rams in a 36 gallon tank with cherry barbs, harlequin rasboras and some hillstream loaches and they have never been overly aggressive with the other fish or nipped their fins. While they are breeding they do become more aggressive but this just means that they chase away any fish near their eggs. The chase doesn't last long at all and it isn't done very aggressively either. I've never seen any nips in the other fishes fins.
If you want your tank to be successful I'd suggest putting in a lot of plants and wood. Sometimes the male ram will get slightly aggressive towards the female ram and it's best if she has some hiding spots.
Hope this helps!