There are many reasons for grafting a fruit tree onto a selective rootstock. Some of the benefits are:

1. The root stock is frost tolerant.

2. They are disease resistant.

3. They have some other characteristics such as they are a dwarf species or fast growing.

4. You don't have to wait as long for fruit production as you would if you grew it from seed.

The grafted portion of the plant, or donor material, will take over the tree and produce fruit exactly the same as the tree it was taken from, while the rootstock will retain the same characteristics as the rootstock that you grew.

You said your tree is a "super small eating apple tree". Do you mean that it's a dwarf apple tree? If this is the case, then this could be an ideal rootstock to use to graft a cooking apple onto as you would end up with a low growing cooking apple tree.

You also said that your tree is "unbalanced", but I'm not too sure what you mean by that? If you mean that you would like to fill in your eating apple tree with grafts from a cooking apple tree and have two types of apple on the same tree, then this is possible. But, it's not good practice to do it this way because you will have problems with the original (eating apple) tree taking over from the newer graft. The best way to do it would be cut your original tree down and use this as the rootstock for your cooking apple scion.

Regarding the best time to graft, it depends on the type of graft you are using. Chip budding and T-budding are done from the middle of summer to early autumn. Whip and tongue grafting is usually done in late winter or early spring.

Hope this is of help?

Thanks Myron, very much. Will reply properly tomorrow.