As Snark said, I find it odd that your trainer isn't helping you out here. Have you spoken with the english trainer about wanting a horse?
And with size, there are different types of Welshes, I assume your talking about a Welsh B? There is A, B, C's and I think D's. A being the smalles hand restriction and C having none at all.
A B, is usually around 14h usually not described as being a horse.
I've worked with mixes before and they were fine.
The thing with ponies, why most people consider them mean, is from my personal experience people who own ponies let them get away with more. The bigger the horse the more experienced the rider.
My old schoolie, Tina, knew that she could get away with certain things. She was smaller so inexperienced people rode her and every now and again I'd have to ride her to staighten her up.
I don't think that it's "welsh's" in particular that are mean, but rather ill-mannered ponies that have a bad rep.