I was wondering what is the best  overall supplement for a horse on mostly alfalfa, small amount of grass hay and a little pasture. Looking for something to meet his needs as he does not get any grain.

I have literally used formula 707 daily essentials since the 1980s. It's one little scoop a day of alfalfa pellets and has everything a horse is said to need for the day. It's very economical , puts a shine on one etc. they do make other formulas as well if you need those but daily essentials is just that , essentially what they need all in one little scoop . 

Daily Red fortified :)

MVP Exceed 6 way. I can't believe how good my horses feel and look after 30 days. I also sell this product but sought them out to sell because of my personal experience, I only sell things i use and believe in. www.vital-equine.com

California Trace Pellets (minerals)

And I forgot to add:

 Mvp's (Mega-Mag and Mega-Cell) I use the California Trace on my IR gelding and the Mega-Cell on my mini's and other 2 geldings. 

Platinum Performance or Equipride. Have used both for years. 

Dynamite and with a soft/thin soled horse I add their SOD Blast. Only thing that worked for my horse. His hooves are time tightest with no cracks for the first time in many many years. 

I'd say either Equinity XL or Cur-ost Green

my horses are on a straight forage diet - i feed them flakes of alfafa morning and evening

in the evenings they get 1lb od Purina Amplify with their supplements...

FINISH LINE ULTRA FIRE is what i have used for years 

Horsetech High point 

Horsetech Arizona Copper Complete

Equifeast

California Trace

Madbarn

These are my favorites that I recommend to just about everyone.

I top dress my horses feed (he gets fed less than the bag says he needs for complete nutrients) with a .25 lb per day ration balancer so he gets all the vitamins and minerals he needs. I use Sentinel Topline off of Chewy. A bag is $35ish and it will last at this rate for months. 

Remember, Ration Balancers are not really meant to balance the diet for vitamins and minerals. While they may have a insignificant vit/min package, that is mostly marketing. In general, Ration Balancers are formulated with soy to be very high in protein to "balance" a low quality roughage protein shortage. They do not balance the whole diet. Great marketing, but not necessarily what most people think they are. There are some good vitamin / mineral products out there. But, Ration Balancers typically are not them.

I have my gelding on 3lbs a day of grain and triple crown complete ration balancer. He's hypp n/h and has been doing good on this balancer but now you have me wondering if he's getting the needed vitamins and minerals since he doesn't get the recommended amount of grain. I try not to switch up his feeding much to not cause an episode. He's on Exceed 6 way and DAC oil..what are your thoughts?

Do they have access to salt, mineral blocks?

in reality you cannot properly balance your horses diet unless your hay and pasture is tested and then fill in the gaps . The world balancer is definitely a marketing thing for all these companies and supplements.  It's a best guesstimate at most. I don't test hay or pasture but I do use a RB with a few pounds of a concentrated feed for a few extra calories. It works well for my easy keeper that needs just a bit more. 

Completely agree

Did anyone try giving Formula 707 not only to a horse but also to a dog? I read that some vets prescribe it to dogs, and it seems to be convenient to buy one supplement. I'm also going to check petsylife and look for the places where can I get it in my area. Maybe I'll get more info at the store. 

CEP.  Hoofrx if I had to choose just one. Impact IV would be next.