Sorry, still a bit new to all this (Hence the handle )

I currently have two Red Cap Oranda's and a Hypostomus Plecostomus (Common Plec) in amy coldwater set up which has been at a constant 75 degrees.

I am aware that some species of tropicals would live happily in that temperature of water. Which species could I purchase to complement & live happily with my current stock??

Thanks in advance

I have kept goldfish and tropicals together in the past and sooner or later you will have problems, if you have determined to have some, corys make good tankmates as they clean up the gravel and left over food that your goldfish leave, on the minus side I think that goldfish are piggies and dont leave much food anyway, I used to feed plec waffers and and bottom feeders algae tablets, which my oranda used to suck up whole.

Best to keep tropicials with tropicals and coldwater with coldwater.
As fishyfeet says, you will run into trouble sooner or later if you mix the two.