What type of clown are you keeping now? Is it normal percular/common clown? They will be fine in a school, as for the size of the anemone, a big one is sufficient to house all of them.
Do note that anemone is an invert that requires good water parameter, they requires strong lighting as well. If you are using 4 x T5 HO or >150 watt MH light, it should be sufficient. If you are using normal t8 light tubes, you will have to upgrade your lighting before getting an anemone.
The anemone will moves around until it found a suitable spot for itself, sometimes the spot that it likes will not be a spot that it can show its intense beauty for the tank, you might have to consider.
Yellow tail do you mean damsels? The anemone will eat fishes that go near it, but damsels should be fast enough to avoid their tentacles. Bubble tip are not so dominant, if your get carpet, it might kill more slow moving fishes. From my previous experience, my carpet have killed some fishes like fox face, butterflies and even purple tang.