1. well, as you probably know the max lvl is different from normal card and fused Supuer idol card. So with your lvl 6 card, you would still need to fuse with another same card again to get max lvl. So just keep the already fused Super idol and feed the lvl 6 one.
2. yes the only way to lvl up cards is by feeding other cards.
Also some thing people might not know about:
-When you do special practice(fusing the same cards to make super idol) Which one you pick first matters.(your base card)So for example, you have 2 of the same N cards, but one is lvl 10 with 25 bond and one is lvl1 with 0 bond. If you pick the lvl 1 with 0 bond as the base card and pick lvl 10 one as the one getting fused, you will get lvl 1, with 0 bond super idol. But if you do it other way around, you get lvl 10, 25 bond super idol.
-when you lvl up card by feeding, make sure you feed the same color. They give you extra exp.