[Anzipane]

The best way to achieve a custom class selection is setting the player in spectating mode when he comes to the class selection:

pawn Code:

public OnPlayerRequestClass(playerid, classid)
{
    TogglePlayerSpectating(playerid, 1);

    // Make your own class selection...

    TogglePlayerSpectating(playerid, 0);
    SpawnPlayer(playerid);
    return 1;
}