Roosters are a must if free ranging but I'd only have one in a static run if I wanted to reproduce my flock. One hen will usually step into the flock master role if there is no rooster. If you decide to get one, the usual ratio for rooster and hen is 1:10, though a much older rooster will be just right for 5. A younger one can/will wear the feathers right off their backs and can cause them to be too nervous to lay.
Introduce them by sectioning off a part of the pen and putting your new bird there for a week. Let them see him, hear him but no physical contact yet...then blend. It
should go alright if he is older and there shouldn't be any dominance issues, but if younger, the older hens may kick him around a little until he gains maturity. Depends on the breed, the age, the flock order, etc.