Mangosteen has been grafted on other garcinias, but they have not done well long term. The countries that grow and graft them, do so on mangosteen rootstock for better success. This doesn't mean that you shouldn't take the chance though.
If I were to do this all over again, I would keep some seedlings to grow out and have some separate ones to attempt your grafts. You might just find that the seedling won the race after all. Hawaiian Tropicals have nice sized seedlings and these would be a good place to start. These would also give you several years head start over growing your own from seed. You can order scions from Oscar I'm sure once you mastered keeping the seedlings alive. Anyways...just my two cents.