Learning Curve

I recently received a set of Easyboot Gloves for Hoss.  I was very careful about the process. I trimmed and measured his feet.  Based on my measurements, I ordered a Fit Kit.  When I got the Fit Kit, I tried them on, and came up with sizes.  Oddly, although based on my measurements he should wear a size 2, I wound up ordering 1.5s for his fronts and 1s for his hinds. With the Fit Kit, it looked like those sizes were perfect.  It even looked like 1s would be too small for his fronts.
So today I took him out for a "test drive" in his new boots.  We went to the steel bridge, where I knew I could test the boots over very challenging terrain.  I wouldn't call it a disaster.  Although the fronts stayed on just fine, it was clear when I removed them that they were just ever so slightly on the big side.  They certainly shouldn't have contained that much water if they fit right!  The hinds, though.  I had to fix one early on in our ride.  Later, I realized it had come off again, only this time, the gaiter was ripped halfway off.  After crossing the water, I let Hoss gallop.  Both hinds came off, although remained attached by the gaiters to his pasterns.  On one, two of the screws attaching the gaiter to the boot had pulled clean through.
The conclusion is clear.  Although removing the boots by hand felt something like trying to remove shoes without the benefit of shoe pullers, I've got the wrong sizes.
When we returned to the trailer, I pulled off the front boots and tried the hind ones on.  With the little bit of lubrication from his feet being wet, they went on well.  I now feel that the 1s are more appropriate for his fronts than the 1.5s, despite the fact that the 1.5s did not come off nor cause any trouble.  On his hinds, the 1s were a disaster.
I'm debating taking Hoss to the Eastern Mojave Scenic endurance ride this week.  It's a multi-day ride through the Mojave desert.  He's now effectively fully recovered from his hock injury, provided we don't do a whole lot of collected work.  My plan had been to go entirely in boots.  Now, that's clearly not possible.
What to do?  I'm thinking about just going ahead and nailing a pair of shoes and pads on his hind feet, and using the size 1 boots on his fronts.  I'll have to take him out again on this set up before we go, just to be sure I like the feel of it.
In the mean time, I'm going to order another Fit Kit, and see what the heck those hind feet really ought to be! I'm thinking there's a bit of a learning curve on these things, and I need to look a little more closely at what I'm doing.  I should have gotten another Fit Kit before ordering the boots in the first place.  That was a poor choice on my part.
If I go to Eastern Mojave, which I hope I can, I'll be shooting for 3 days of 50s.  It's the last chance to go to a ride I have before my surgery, and I don't expect to be truly back to fighting shape until Descanso in June.  There've been some serious machinations in getting the opportunity to do this ride without spending money, so I really, really want to go!