1. I one grafted a orange seedling with lemon scion. At that time, the seedling was 10-15cm tall, with 5-6 leaves. In one year, the grafting line grew to about 30 cm above the soil level. So the rootstock part will grow both in thickness and height.

2: It is proven that using multiple rootstock for the same scion will make the scion grow faster, with shorter internodes. Combining different rootstocks offers best results, since roots as whole are active for longer period. (there is a scientific report or a book about this, but I must re-search-it)