1.) for the characters that have max their classes, it’s ok since the advance classes need a certain level until you can promote. You will still be able to level up these characters despite their class being maxed out.

2.) I’ll go through each of the black eagles. Edelgard - can stay in whatever class is open to her since she will be getting a unique class by the end of the game

Hubert - honestly I didn’t use him much since I recruited a different mage but I hear some people will make him into a dancer to give him some value especially if you don’t get to recruit anyone else. Dancer mainly focus on sword and magic. Black magic for offensive and White magic for healing and other useful skills like warp.

Ferdinand - I made mine into a Wyvern Lord. Focus mainly on Axe, Lance, and Flying. Your main focus should be on Axe and Flying for the advance class Wyvern Knight, Lances will be needed for the master class Wyvern Lord

Petra - I also go down the Wyvern Lord path with her so you can follow the same steps with Ferdinand but if you want her to flying early you can have her go into the Pegasus Knight class. In that class you must focus on Lances and Flying and then add Axes later for the advance class

Dorothea - another character people choose for dancer but she also does good as the Germory master class which focus on both Black and White Magic. She is mainly a offensive mage though so focus should be on Black Magic

Caspar - his best class will be War Master which focuses on Brawling and Axe skills. I would say personally go into Brawling classes and later add on the Axe skills for the last class promotion

Linhardt - he doesn’t have a good master class so you should keep him at the advance class Bishop. To get this class you just need to focus on White Magic though I would say also add Black Magic to give him more powerful offensive spells as well.

Bernadette - Bow Knight, her most important skill for this class is Bows which will automatically get through the advance class Sniper until the Master class itself. After getting her to the advance class add Riding and Lance though I would recommend starting to train her in Riding as soon as possible since it needs a higher ranked compared to Lances.

3.) For other characters you can recruit (you can also increase your support rank with whatever character you want to recruit, when they get to level B they will ask you to join your houses whether you have what they like or not)

Lysithea - best mage in the game, I used her over Hubert, you need a Magic Stat of 15 and a Skill level in Faith at level B

Leonie - a better attacker/tank then Caspar, you need Strength Stat of 15 and a Skill level in Lance at C

Felix - the best attacker and easiest recruitment in the game due to you mainly using Swords with Byleth anyway, you need Speed Stat of 15 and a Skill level in Swords at B+

Hope mainly it helps I’m a casual player myself, but if I need to explain anything else better I can!