I'd decide what you want in the offspring, and go from there.
You say some fun breeding, and your hens are mixed already. So, what is "fun" for you? In my mind, that would be something like a Silkie rooster, that could give you some cool offspring. Or maybe a Polish, those crests are partially dominant and you'd get some funky chicks. Both those breeds are small-ish.
It's going to have a lot more to do with the individual rooster than an overall breed, though. I've had roosters in breeding pens as trios (one rooster, two hens) and the hens have been just fine. I've had a rooster that free ranged with 2 dozen hens that psycho stalked two specific hens and tore the crap out of them . Pretty much all we can say is, look for a rooster you like and give it a try.
I agree space is going to be a key factor in how well everyone gets along. So will the age of the rooster. A young cockerel may be put in his place by your older hens, but he may make life hard on the younger pullets. An older, more mature rooster that courts the ladies is often a better choice, if you can find one.
Most bantam roosters can fertilize a large hen. Fertility may not be as good as a large fowl rooster's, but he should be able to get you some chicks on the ground.
Or, another thought is to source hatching eggs when your hens go broody. I know it's not quite the same as hatching your own, but it would avoid the issue of keeping a rooster full time.
Although, hatching will give you roosters to deal with, eventually......
Edited by donrae - 12/12/16 at 10:44am