So what will the new class abbreviations be?

Some of those names don't need to be abbreviated. People don't abbreviate "Ranger," or "Fighter," when talking about my characters, and those are six and seven letters. Captain is seven, Viper is five. They're fine as they are.

Valk is an obvious abbreviation for Valkyries, they exist in other games and it's pretty standard as an abbreviation.

The problem words are Infighter, Gunslinger, and Buccaneer, I think. They have no clear abbreviation for them, and they are pretty long to type out.

I would just go with "gun" for gunslingers, since when I'm discussing archers I don't hesitate to say "bow" to mean Hunters, Rangers, and Bowmasters, and "xbow" to mean Crossbowmen, Snipers, and Marksmen. Infighters and Buccaneers - I'll just go with whatever other people start calling them.