I know plenty of folks breed younger, but personally I never like to contemplate breeding till the horse is fully grown. At this point, it's unlikely that your mare will sprout up or develop any serious conformation issues, but she might. She looks okay now, but if she doesn't get much taller, she'll likely end up a large pony. I've never registered a horse through the AQHA so I don't know if they have height requirements. At the very least, if she doesn't get much taller, she could pass her shorty genes onto her foal. I don't know if that's something you're concerned about, but it's something you should consider.
The rest of her looks decent to me. I also don't know what Blind Horse Enthusiast said about the hindquarters. They look fine to me. Admittedly these aren't conformation shots, so perhaps if she were stood up properly I'd see what Blind Horse sees as well. Or perhaps she's just standing wonky. I don't know.
I do know that I would never breed a mare who wasn't fully grown. 6 minimum. And I'd want her to have a job, be at least moderately successful at it, be registered, have great conformation, and have a good mind. I'd also want to have a job in mind for the foal. What's your aim here? I'm assuming, from your original post, that you're looking for a catty cutting horse with work ethic. That's excellent, you've already got a plan in mind. I know cutting horses tend to run on the small side, so even if she never grows another inch that might not be a terrible thing for you. For a catty cutter with work ethic, I think your cross would be pretty decent. All the logic seems sound to me.
One more question you ought to think about before breeding - will you still want a cutter out of this mare even if she shows neither the desire nor the aptitude to cut? Obviously breeding is no guarantee of ability - I've seen reiners, barrel racers, ropers, jumpers, all bred out the wazoo for a specific thing, who either hated that thing or were crap at it - so even if this mare can cut the legs off a fly from 100 yards, her foal might not want to or be able to. Or the opposite could happen - the mare could turn out crap at cutting, or hate it, and her foal could be a star. Or, neither of them could like it or be good at it, or both of them. Anything could happen. So. What will you do if the mare hates cutting? The foal? Just something to think about.