Note that it's not necessary to have matching manuy type but it's still recommended. Using unmatching manyu will just give less Exp to your girl. So, sometimes, you may still want use every kind of manyu on a girl for some reasons.

yes but 50% increased efficiency can make a world of difference. I know it can be tough starting out as you want to be able to clear E-8, but you end up paying more in the long run.

Also if you do not need 10 to get to the level cap (5* 60 before evo 70 after etc) then only use what you need. No point wasting them.