Hi everyone

I've been keeping fancy goldfish for 6 months now and have really been getting into the fish keeping hobby, I have an old 21 litre (about 5.5 gallon) tank that was sold to me to start my goldfish hobby (before I knew any better, they're in a huge tank now) and over the last month or so I had been cycling it, knowing I'd be putting a fighter in it eventually.
I only wanted to get a fish that I'd instantly fall in love with, and after a few weeks of searching, it happened earlier today. They had a few strange breeds that I hadn't heard of before there, dumbo and giant. I went for the giant (the dumbos appeared sickly) thinking it was like any other short finned fighting fish, only with a slightly larger and thicker body.
After bringing the fish home (haven't named him yet) I tried looking up his breed but couldn't find too much except that they could grow to about 5 inches long and are peaceful..
So does anyone know about this breed? Are they different to any other breed of fighting fish and do I need a bigger tank? Also, will he be alright to put in with my goldfish if his tank is too small? I'll be happy to get a bigger tank if needed, just limited on space at the moment.
Sorry about the long post..