First of all, you don't graft a seedling onto the rootstock. That defeats the purpose, which is to continue the production of a particular cultivar, which would have originally been a seedling, but has some attributes that are desirable and you want to maintain.
So the root stock that you would graft scions of the desirable attributes onto, are just seedlings. They are grown to a certain size, then the twigs (scions) are cut from the cultivar, and grafted onto the seedlings.
The plant you use for root stock is just seedlings of any Tamarind you have that shows good adaptability to your conditions, ie: drought tolerance if you live in an area that doesn't get much rain, or possibly height so the adult tree stays small.