I have only been reading this thread, but wanted to say that if the red roan colt is the one that you are thinking is not conformationally pre-disposed to stifle lock, I would implore you to think again.
He is quite straight behind, and sickle-hocked from the looks of it.
I am not saying to geld him, just hoping you'll have a look around at horse conformation before you close your eyes due to poor wording on this forum.
No one is singling you out (I do not even "know" you, but I applaud you for looking for information, and hope that you can find it in your heart to realize that yes, he IS built to lock up, and if he's built that way, his foals will be, or if they're not, it will be pure luck that they aren't.).
My thoughts are with you...(see the gelding in my avatar...he is pretty well "camped out" and though I hope he doesn't lock up, I suppose he could although he never has. I just knew he was not needed in the breeding world when I look at what "good" and "Sound" conformation is). It's not an easy thing to admit, but it is far easier than the heartbreak of having more problems down the road with babies.
Right now it's a known quantity...and I wish you the best of luck. Hopefully just keeping him trimmed properly will keep him sound. Others are not so lucky and it's sad to see one locking up. they simply don't understand why they are so limited.
Please accept the advice given here without hostility...no person here is trying to thwart your future breeding program, but rather to head off the tragedies many of us have seen firsthand in a genetic condition we closed our eyes to in our OWN ignorance and inexperience. That's all that I see here...
Another quick addition is that at least in halter, I think your little colt would end up not doing "so well" due to his hind leg angulation, but that is not to say that he could not do well at other things provided he's sound for it.
I am trying to be helpful in sharing information that I can see even in my very amateur observation, that it is more than a matter of opinion, his hind leg angles combined with what you've said about his condition prove that stifle lock is something he was made to have. The farrier trim may not have helped, but it didn't create the condition, only worsened it.
I am so sorry about that...I know how it feels to realize something like this. I do (and many of us do). I wish we could convey that to you without you feeling defensive.
Liz