If you add addballs, you need to add the addballs to the foal LNZ too but you can make their size -99 so they won't show up on the foal, but if you do want to them to show on the foal then you'll need to size them down most likely. I've only so far sized them to -99 so I might not know what I'm talking about when you want them to show. I think you can do that either directly in the foal addballs or in ball size override in the foal, but I always just use the addballs section.
Or, if you remove addballs instead of omitting, you'll need to do the same on the foal. So far that's all I've done that's messed up my foals, had addballs not matching between adult and foal. I had some crazy project ball issues but I've only come across those working with the draft versions and leg fluff, and actually deleting things instead of omitting them. I can't remember exactly what I did there but I know I had to delete/change stuff in project ball because the fluff is tied to the legs in there. And then I had to re-do some linez. I'm pretty sure if I'd just omitted the fluff and not deleted it there would have been no issue though haha. I learned a lot! But seriously though, save your sanity and just omit stuff if there's something you want gone lol.
I know there are other issues than what I've come across, and mine is pretty basic and you may have already known that lol. I'm interested to see what other fixes/preventions there are, because I've seen some foals that look like they're missing linez and some other crazier things in the old days that I'm not sure how it happens. For me, just making sure addballs match between the two has worked for everything.
The lines missing is actually a fix Daveena taught me and my mod already hit that XD, it also hit the "I want part of my neck to look like a chunk is missing, both of which I've had to fix already, but thank you so much, this mod means the world to me getting it to work for foal and adult =3 it's for Cyove and her daughter as I said, now all i have to do after fixing the foal AGAIN XD is make a texture, lord save me
maybe, I've already run into trouble, i got the addballs working, than i fixed the foal and there was a line from one addball to the butt, i thought this was because i added an extra line, and so i deleted it, now the foal is showing the addballs but it fades when the animal reaches adulthood instead of showing the adult size GRAH