Hi There

I myself have a Golden Wonder Killi
To answer some of your questions. These guys will get to about 4" when full size and they do grow rather quickly. I've had mine for almost 4 months and he's full size.
Food wise, mine eats just about anything, from frozen brineshrimp and bloodworms, shrimp pellets and flakes. Doesn't seem to be picky about food.
The males will be a bright yellow with red edging on their fins and the females are more of a dull yellow/tan colour.
They are like egg-laying guppies, meaning they breed as often as guppies do, but just lay eggs.
As far as keeping other types of killis with these guys, I'm not too sure on that. I've heard that you need to stick to one type/species per tank as they all have different tank requirements and may be territorial towards each other. I don't have experience with that though, so hopefully someone who knows more or has had experience with keeping different types together could help you on that one.
Your tank size is fine for these guys. They almost always stay at the surface of the water and occasionally will swim to the lower parts of the tank especially if there is food there.
They are peaceful with other fish, so you shouldn't have a problem at all.
I have mine with angels, neons, clown loaches, gourami, rummy-noses, harlequin rasboras,and pleco and I've never had a problem. He just minds his own business.
They are great fish and I love them. Good luck and if you have any other questions, let us know! :dance