I'd just keep going with ARC for the time being, especially since you're itching to get your mount and retainers. Those both unlock by progressing so far in the main scenario, I believe between levels 22 and 25 give or take. So you'll probably want to get your archer at least to the point where those unlock before switching over to PGL. Your PGL will start at level 1 of course, so you won't be advancing the main story with that class (unless you level enough to surpass your Archer).

You won't get exp from main story quests on any secondary or further classes, however, all but your highest class gets a 50% exp bonus.