By the way for any FE: Awakening experts. Is there a benefit like extra skills for keeping a character a base class until level 20? Like if I let Chrom hit 20 before making him a Great Lord.
Depends, if you are switching from a mage job where you got X skill and you will never be magey again it can help to level that up to 20 for better res and whatnot, stats that you otherwise miss out on when you switch to physical jobs. You lose a chunk of stats but you also keep a certain amount and moreso than if you don't go to 20.