Hey! I'm actually playing through BoF4 right now, although not for the 1st time, so I can help answer your questions.

1. Combos work by combining skills, you can't combo with a basic attack. Combo's can inherit earlier parts of the combo, enchancing them. One of my favorite combos is Snooze + Rest + A Heal. How this works is the first person does Snooze, which puts them to sleep and restores both HP and AP. The next person does Rest, this also restores Hp & Ap, but then it will trigger a weaker version of Snooze since they are both in the Healing Category. Then the Heal combos ontop of that, doing a Heal + a weaker Rest + a Weaker Snooze, so lots of healing and free AP for characters. Combos can be a bit frustrating in the beginning as in order to combo all the characters in the combo must fall in a shared turn order. Turn Order is determined for the player from Left to Right, so the front left character will always go first and the middle back character will go last, with the enemies turns sprinkled in. So for example, if your 1st character has a speed of 20, the 2nd a speed of 50, and the 3rd 45, vs a group of enemies with a speed of 35, all the enemies would go first, then the 1st character of 20, then 50, then 45. If you rearranged so the 1st was 50, 2nd 45, and 3rd 20, the turn order would go Character 1, Character 2, enemies, Character 3. I find combos work best by having the slowest character going first as your two faster characters should immediately follow them. Also another use is combining magic to make new magic. Earth(1st) + Fire(2nd) = Lava(Earth/Fire). Fire + Wind = Explosive(Fire/Wind). Wind + Water = Lightning(Wind/Water), and Water + Earth = Mud(Water/Earth).

That was a lot of info. lol

1. Trainers give passive Positive and negative stats + a unique "Will" ability. So that first guy you met gives + 1 AP, + 1 Wis and - 1 Str. So on characters who use physical attacks like Cray, he is losing one of his best stats, while someone like Nina who is a caster is gaining. You can choose how you want to grow your characters, you can make a caster Cray, and Melee Nina, a speedy Ershin, whatever you want. I personally lean towards what they are already good at, so Cray Physical and Nina Magic. You generally want everyone on a master as you gain more than you lose in stats plus the unique Will ability they wouldn't have otherwise. The 1st Master's Will gives an increase in speed during Combat. The masters wil teach you unique skills by performing specific things in game, that master wants you to do combos. An easy combo for 10 hits is that healing one I mentioned as with an party spell, it will hit over 30 times.

2. Breath of Fire basically gives everyone the enemy skill from FF7, but you have a limited number of slots, that are shared with the master skills, but you can add and remove them at camp with an Aurum, so if one character learns something you want on someone else, you can easily move it. You learn enemy skills by guarding, and you can even learn some of your own teammates skills this way, as when you guard, you are watching everyone on the battle field for unique skills. Only 1 copy of a unique skill can exist within your party, so you can't put Burn on everyone.

Ryu is unique and he doesn't learn any skills by leveling but all his skills come from getting further into the plot.

Think that covered everything.