There is a bonus for element and a bonus for cards.
If you chain sac with different elements, it takes 14 commons to max one.
If you chain sac with the same element, but with different cards, it takes 11.
If you chain sac with the same card, it takes 8.
Same goes for sacrificing for UCs, but not that much.
2 Maxed Cs of the same element of the UC you are sacing into, gives you a Lvl 27 UC, other than that you get a Lvl 26.
As for skills, if you are planning on using on some card that you want to evolve later, don't bother, because they reset on evolve.
After you have your Final Form, just be sure to Sac Skill UCs into it, and not random UCs
After you Max it, you can still sac cards to improve skill level.