I'm not much help with the joint supplement, but in regard to the Enrich 32 vs. SmartVite, it depends on what her whole diet looks like. Is she getting any hard feed in addition to the E32? Is she getting 100% alfalfa, or is she also getting some grass hay? If she's still getting mostly grass and only a little alfalfa, you don't need to switch ration balancers, but you can also look at Enrich 12; it's the same thing as E32, except formulated for horses on an alfalfa diet.
I don't think there's any reason to use both E32 and SmartVite- if she's getting at least the minimum per the feeding directions on the bag of E32 then she should have her bases covered in terms of vitamins & minerals. However, if the SmartVite is cheaper, then switching might make sense.
It sounds like you're trying to put a little more weight on her, and that her workload will be increasing in the near future, so some additional calories are probably in order, especially if you end up switching to the SmartVite. Beet pulp, rice bran and alfalfa (pellets in my case, but hay is even better) are my first go-to's for adding calories.