My flock is a mixed wool flock, soon the be even more diverse (clun mules, scotch mules, purebred tunis, suffolk cross lambs from those ewes, as well as some corriedale lambs, soon to add a few cheviots and some Gulf Coast natives). The nice thing about the mix is that the individual sheep range from light to medium (none terribly heavy) and I can mix things around as needed. I prefer working a larger group, especially for the youngsters because I think they can learn a lot by working bigger groups. Recently we have been working young dogs on the ewes and lambs. It's an easy way to dog break the lambs and it is good thought-provoking work for the youngsters. This is a group of about 30, give or take.
There is also a 50-head flock of dorpers on this farm. They are worked less often because they are really only suitable for trained dogs. If I had time, I would work them more to make them more suitable for the youngsters as well. They're lambing now though, so any of that will have to come later.
There is also a small flock of Boer goats on the farm, as well as a small flock of geese and a large flock of chickens (as well as guineas and peafowl). The dogs who will work poultry are allowed to work them. I will use a well started, sensible youngster on them, but don't routinely train on them--mainly just work them when they need to be moved for some reason. The chickens are put up at night, so sometimes need rounding up to be put to bed. I use the trained dogs for that. One subset of the goats is used for trying out puppies and starting youngsters. They have their advantages (won't run over a person) and disadvantages (don't flock as well, too smart for their own good). I generally move youngsters off the goats and onto sheep fairly quickly (or start with sheep if the youngster clearly doesn't like working the goats).
Re: the sheep. I prefer wool sheep and that's mainly what I work with. Now that we have a shearer nearby it's not so much trouble keeping them. The hair sheep belong to the farm owner. The nice thing about hair sheep is they are much more heat tolerant, which is important here in the south. But I don't think I'd ever personally go over to just hair sheep, unless I absolutely can't help it (like Mark's situation).