Introducing a new rooster is not a problem as hens and roos have a separate pecking order, and the girls will not see him as a threat. Personally, i would get one at around 6 months old at least. Assuming you are not wishing to breed from him in the next 3-4 months, it may make more financial sense to wait until late Jan / early Feb before you get another rooster (otherwise you will be paying for feed etc for a few months with nothing to show for it) - that way he will become a member of the flock in time to give you fertile eggs come spring. I guess it depends on why you want a roo - if its for protection of the girls, then go ahead and get a mature roo now/
CT