When making carbonated water I just set the regulator at 10 lbs. or so. That's not nearly as carbed as the stuff you buy but the kids like it and it doesn't come gushing out of the tap too fast for them.
As far as the ginger beer being gassy to start, I believe it's going to come to equilibrium in the keg at whatever serving pressure you keep on it. I would think that you would just set the pressure to your desired carb level and leave it at that. That pressure is probably higher for ginger beer than homebrew. I think I would probably start at 20 lbs or so and see if I like it in a few days. If not, kick it up or down as you see fit.