I have four horses and, without writing a book, am in the same predicament as you.
The harder I try to apply the KISS principle (Keep It Simple Stupid), the less successful I am
At the moment everyone is eating different until I use up what I can't buy anymore and also what I tried on two horses and boy, it's not working.
My next "trick" is to try McCauley's 10% because it is soy-free and does not have grain in it. I have to drive 38 miles to buy it but that's close compared to what I was driving.
If you have a McCauley dealer within your driving distance, this might be something to try and the price is good. McCauley's® M10 Balancer
Regarding arthritis meds, I only buy products that are pure. That way I don't worry whether or not I am over-doing or interferring with somethign else.
I buy pure Chondroitin/MSM for two horses and pure Hylarin/Boswelia for the 25 yr old with EMS, hind gut ulcers and lipomas in the GI tract.
The 25 yr old is such an internal train wreck that he only eats rice bran, Omega-3 Horseshine, chastetree and Tuttles Liquid
747 vit/min supplement. He's now a hard keeper and I have to feed the rice bran three times/day because he can't eat 3-1/2 pounds at two feedings without colicking.
Just some off-center alternatives for you to mull over