I think you're on the right track with taking it easy and working up to things slowly. I personally would never ask a single 32" mini to pull 500lbs (my 33.5" gelding had a heck of a time hauling more than 250lbs cross-country) but then again my gelding was not built very well either. Still, start out ground-driving and build his fitness then watch him and let him tell you how long he can go and how much he can haul. They're all different!
On a cart with adjustable balance, I prefer to start with the shafts at the correct height for the horse and type of cart then change the balance until it's correct at that height.
Only for a show cart or jog cart would you change the shaft height to change the balance.
It might be a bit much for her right now if she's finely built and has never been out of an arena, but if you keep building her up slowly she'll be fine. They usually love going cross-country! My boys are both 33.5" and once Kody was conditioned for the MiniCrown he had no trouble pulling it up even very steep hills. Turbo is a baby (3 years old) and has only been hitched a few times and right now he thinks even the Frontier is a heavy monster and "impossible" to trot with when he's tired.
Funny thing though, when I hitch him with the other gelding as a pair he can go all day with great enthusiasm! Sometimes they don't know what they're capable of until you show them. Once he's conditioned Turbo will be the more powerful of the two geldings by far; he just doesn't know it yet.
Leia