I'm going to second, third and fourth the advice you've gotten here. I agree that the collar and hames is the best ergonomic model, period. You can't beat the simplicity and efficiency of the design. They are correct and awesome for any sort of draft work. However fitting them to a mini SUCKS!!
I can't personally stand the look of the buggy collar Chimacum sells for minis (I don't like the dull leather, the width of the collar especially at the top, the lack of shape and the fact you can't get the hames in brass anymore) but I bought one anyway to try out and I swear Kody's entire neck disappeared into it. Now I could have lived with that if it had worked nicely for him, but he told me immediately that he was NOT fond of the anchor around his neck and I ran into a little problem with his conformation...he's got a lowset neck which is wider at the bottom than the top when he's standing so a collar that fit when he was relaxed would have been rolling around once he started using his topline, and one that fit when his neck was properly arched was way too tight when he's standing!
He also has narrow shoulders as far as the meaty surface area available for a collar to sit on so that would have been a problem as well. I could also see that conditioning his shoulders to regular fast-paced work against such a hard surface was going to be a painful process and I decided on the spot that it wasn't worth it. If I were going to do any sort of draft or agricultural work I would buy a collar, but then again the horse isn't going to be trotting at speed cross-country during that sort of labor so their shoulders wouldn't take such a soring.
Two other things to consider are the draught arrangement your carts have and how the end of the shafts are going to interact with your collar. Neck collars should not be used with horizontal draft like most EE's and show carts have! Even if you lower the singletree, you may then find that the shaft tips get hooked on your collar or short tugs and cause all sorts of problems. Blegh! I've watched friends go through that. Funny thing, even the ones who drove in collars for a few years have gone back to the new shaped breastcollars as being the better option.
If you don't already have it, buy yourself a copy of Barb Lee's
"Understanding Harness" and then you'll have a better idea of what you're looking at and why folks with easier-to-fit horses love neck collars so much. I'd love to have one but for me, with my individual minis, it just wasn't feasible with the dollars I had available and the products currently on the market.
Leia