Ah yes, I did try it out and then promptly forgot to report it here. Apologies for that.
Custom battle works fine all horses are present with correct textures.
New campaign starts up fine.
Continuation of old save works fine and the new horses (bodyguards) show up when fighting Gondor.
Yet to do:
Test new campaign with Gondor and fight battles.
So far no issues, and I like the look of the horses.