You have to be careful though. Since you can reclass as many times as you want provided the requirements are met, you can keep growing in new classes. The game keeps track of a unit's internal level, which is the sum of all the levels they've gained in all classes. If you level to 20 each time in each class, it's gonna be hard to gain experience later one.
For instance, my Avatar's internal level is 76. She barely gains any experience anymore, because she's gained so much already. That's 20 Tactician, 15 Grandmaster, 10 Mercenary, 15 Hero, 15 Great Knight, 1 Dark Knight. If I'd gone to 20 in all those, I wouldn't be gaining experience anymore at all. Or not enough to matter.
If you want those skills, you might need to promote and/or reclass before 20.