The only problem with adding an all-in-one like that is that you will be doubling up on things he is not lacking. For example, excess iron can block the absorbtion of certain minerals and vitamins. If you use well water that is high in iron, then you need to avoid supplements with extra in them.
Before adding a supplement, I would have his diet fully analyzed to figure out exactly what he's lacking and if he's getting too much of anything. If you want to learn how to balance the diet yourself, Dr Kellon has a great online course on horse nutrition. www.drkellon.com
There is a lot of great nutritional help on the yahoo equine cushings group if you don't want to do the above course. EquineCushings : Equine Cushings and Insulin Resistance
You should have your hay analyzed, so you know when your horse is getting there. If he's on grazing, have the pasture analyzed too. Equi-Analytical Laboratories - Profiling Feed for Better Nutrition
I would call or email TC and figure out exactly what he's getting daily based on how much of their senior feed you give him daily.
If you're on well water, you might want to get that tested. Your local cooperative extension service should be able to do that for you, or tell you who to contact for water testing.
So, add up all of the nutrients from each source, including your joint supplement, then send that to the cushings group or use an online tool like feedxl.com, and you see exactly what your boy needs to balance his diet. You can have a custom supplement built very affordably by Uckele Equine. I get mine done there. Home page
I add to my base supplement extra biotin for hoof growth, amino acids for muscle development (we don't have much grazing), extra magnesium for my "easy keepers" and moody girls, extra b-vitamins for moody girls, and extra copper/zinc to balance our high iron levels. I also feed 1/2 to 1 cup of flax for good omega 3's. I don't use any feed of any kind though, just alfalfa pellets, a little plain oats, and all the grass hay they can eat.
Since going "all natural" and really balancing my horses' diets I have seen improved hoof/hair growth, shiney coats year round, better attitudes, and better weight (less on my fatties and more on my hard keepers).