You can set the Island Settings default class, but then you will need to use the same class for both teams, then change class after the players arrive when needed.
When set to Teams Split Evenly the players will arrive to island and be allowed to spawn on team set spawnpads, becoming that team.
A player cannot spawn on a class set spawnpad if they are not that class. Only the default class can spawn on the same class spawnpad.
So if you want 2 different classes at the start, make no default class and set Class to Any on the spawnpads.
Then use Class Selector Devices, one for class 1 and another for class 2 and either place them so the spawnpads are within thier Zone, or use the Functions to Change Player to Class on each Spawnpads on Player Spawned Event.
See how you go, let me know if you run into problems.