I’m impartial on this, since I’m a Johnny user. I remember you from all the way back to when GGXX was released, and there was that gigantic thread constantly being posted in. You didn’t stick with Baiken?
Well, I’m totally terrible with recommending characters, so I’ll just answer the other questions.
Instant Air Dashes? All you really need to do is to press up-forward then forward twice, or up-back then back. The diagonal direction counts as a tap forward or back, so it makes the input faster, and you’re already jumping in that direction. The result will be the lowest possible air dash from the start of a jump.
I think Jam has the lowest air dash, usually the height of a character. She’d probably be a good chance to tell if you’re really IAD or not, but the heights of air dashing is pretty character specific.
Now, for Jump Installs.
Basically, during a gatling combo, certain normals allow you to jump afterwards, aka jump cancellable attacks. The idea behind jump installing is to make it so that you can jump or air dash during something you couldn’t do it normally afterwards, like say…May’s FRC dolphin. (a big part of her game, actually.)
There are a few different variations, explained at http://gamecombos.com/games/ggxx/tech/ji.html , but I’ll explain one so that you’ll get a slight idea of it.
Okay, for this example, May’s jump cancel moves are close standing slash, and far standing slash.
May starts a gatling combo on the ground with standing close slash, then standing far slash. During this time, she’s charging up the Mr. Dolphin move (charge back, forward+slash/hardslash), but instead of finishing it with forward+hs, she finishes it with up-forward+hard slash. The jump doesn’t come out, but you fool the game into thinking you have, while May rides the dolphin. You then FRC it, and you can air dash or double jump, something you can’t do normally.
You could’ve also pressed up during the close standing slash, then press the second slash before May jumps, doing the same thing. Same principle, slightly harder to practice.
I hope I explained it clearly. :o