The mini and pony feed isn't suitable for growing minis, too high of sugar/starch, and not the right balance of nutrients for growing youngsters. Despite what the label says, it's not suitable to growing youngsters. Most that trialed it in the early stages, did not stay with it, they found other feeds were more suitable to their needs; especially those showing and breeding miniatures.
A ration balancer is highly concentrated, so not much is needed for adult maintenance; a bit more is needed for growth, breeding and heavy work (not many minis fit into the heavy work category).
Some do need more calories than a ration balancer provides, in that case, I go with senior, largely because it's more readily available than growth in my area; but also because it's easy to digest and typically has lower sugar/starch levels than the growth feeds, especially if you can get a good senior feed like Triple Crown senior. My last foal crop was raised on a ration balancer and Triple Crown senior (well, we started with a vitamin/mineral supplement that balanced my wonky hay, then moved to a ration balancer that balanced my hay, once it became available to me).
Fingers crossed. 
