I think this is really cool, having 5 varieties on one tree. I'm kind of doing the same thing with my Cortland which will hopefully now have 3 other varieties grafted on.

I do think it will be important to prune back the vigorous varieties each and every year to even them out, otherwise 2 varieties will likely tend to dominate the others. Let the weaker ones grow just as much as they want and don't prune them barely at all, but prune the vigorous ones very hard, every year, to maintain the necessary balance.

If the tree is still small, I would allow maybe one or two total fruit on the tree but remove all the rest. Otherwise those pokey growers might never have the chance to catch up, with the energy of the tree all being soaked up into fruiting on 1 or 2 varieties.

I would allow the mystery fifth graft to grow. Let it fruit and see what happens with it. Can't hurt. Just keep it balanced with the rest.

Have fun!