Do your horses need protection on all 4 feet, or can they go bare on the hinds? If they can go bare on the hinds, you only need one pair for each horse going on the ride.
I’ve had two horses who needed boots on the hinds for comfort on the trail (both off-the-track TBs), but the others have been okay bare on the hinds even when they need boots on the front. I’ve also had a few horses who could WTCG across rocks totally bare on all four feet.
You could look up your horses’ measurements on www.easycareinc.com and see which sizes match up. Then either look for Easyboots locally or shop a cheaper site, like Valley Vet.
Before you invest in Sole Guard, a gun, and mixing tips, it makes sense to pick up a pair of Easyboots for the horse you ride the most, and see if you like them. Don’t paint 'em unless you plan to keep them. They should be snug and a bit tough to put on. If they slip on easily, they’ll come off easily, too. The first couple of times putting them on is a PIA, but easier than putting Sole Guard in. 
If you find you like them, you won’t need the Sole Guard, which is a continued expense as opposed to the purchase price of a pair of boots that will last a long time given the usage you describe.
If the L x W measurements work, it shouldn’t matter too much if one hoof is a slight club. A steep club can be a problem for any style of boot. A steep club might be better off shod or totally bare (with Sole Guard) than with a boot that turns. One of mine has a slight club, and he does okay in both Easyboots and Old Mac G2’s–but the G2’s rub his heel a bit and I prefer Easyboot Epics because of that. Every OTTB I’ve ever owned over-reaches from time to time, so I purchase styles that come with a gaiter, such as Epics. I’ve had two purbred Arabians who did wonderfully in plain old Easyboots. What a treat!
If you decide to go with Easyboots and like them, we can discuss ways to modify the boots if you need a slight adjustment.
p.s. The current Easyboot style comes with plastic covers on the teeth (at each end of the heel strap, resting on the quarters), and I leave the cover on. If you purchase an older style on Ebay, the teeth might be exposed and will scratch the hoof wall. This would bother me, but they can be covered with duct tape. The teeth provide grip, but they grip pretty darn well even with the plastic cover.