Alright, I looked at the garuanteed analysis on the supplements you posted and honestly I would not feed all of them together. The electrolyte seems innocuous and if your vet recommended a electrolyte it should be fine.
You shouldn't feed both of the joint supplements together. There's really no need to feed two joint supplements, just find the best one and stick with it if it seems to work. Side note, MSM is a pain reliever so you need to be careful about noticing your horse's pain response as it could allow him to push through an injury/strain that would normally make him show lame.
Next level - Glucosamine, MSM, Vitamin C (horse's produce this in their body from glucose, so unless your horse is old or somehow immune compromised isn't really needed), Bromelain (haven't heard a lot about this, something about blood thinning, which could be problematic if your horse suffers from ulcers), Perna Mussel (couldn't find any research in horses), and Shark cartilage (same as the mussel). If they haven't done studies in horses you need to be careful as obviously a horse's GIT is quite a bit different than either a dog or human.
Here is the analysis for the Missing Link Plus. It has some trace minerals and such that you don't want to over-supplement. Sorry I'm too tired to type everything out like the first supplement. Missing Link Plus with Glucosamine from SmartPak Equine
Sho-Glo: This seems to have everything in it and some of it in pretty high quanities. I would worry about the most about the Selenium levels, especially if you're already in a Selenium rich area or an area where plants hoard (can't think of the technical term) that could cause an enormous problem. Selenium has a very narrow toxic range. To be honest this is not a supplement I would feed with other supplements. But I tend to err on the safe side. Here's the link: Jeffers Manufacturer Information
Hope that helped or at least gave you a good starting point with your vet! Sorry for the book.