Sevs, my personal favorite. Try getting him a female, will really bring out the testosterone in him
Male sevs, as mentioned have longer fins, more colour, and those tell tale worm markings.
As for your Hybrid question, i'd take an educated guess and say its possible. HRP can cross with cons, and i know that cons crossed with sevs are on of the base Xs for parrots (boo hiss)
So it can be done. But the question is, should it?