I've made many batches from concentrate. I prefer to add the fruit in the primary, you have two rackings to clear it up. But adding to the secondary has its supporters. I wouldn't add any sugar. Apple juice will ferment out to around 6-7% without any help. The cherries will push it a little higher. Tart cherries are best, IMO.
My friend Tom adds table sugar at a pound per gallons. I don't, too high an alcohol content overwhelms the apple flavors. There is some brown sugar in the spiced cider, but it's just for flavor
I don't add finings to ciders. I like the added textures.