As far as supplements, I have found that different products/combinations of ingredients may work or not work well depending on the horse. We had an elder gelding with arthritis that did really well on SeniorFlex, but our elder mare (diagnosed with the same thing) didn't get any relief at all (we had her on SeniorFlex for 3 months.) Then we tried a couple other products (each for about 3 months) and very little relief. We put her on NextLevel pellets and she did great; we noticed a significant different in about a month. We then switched to Flex Wafers (cheaper and super easy to feed) and she's still doing great.
It may take awhile to find the supplement that is going to work the best for your particular horse.