They will.
Even though the aesthetics are similar, the old metal ones are both a bit too static to match the more dynamic plastic horses and also too small. Not much, but it's distinctly noticeable. Especially as the legs are not far enough apart to properly fit onto the sadle area and require some cutting on the horse and bending of the knight legs to fit.
It can be done, but expect some work. Also, they will look slightly out of place next to plastic knights, but will look fine as a unit of their own.
I've used the plastic horses throughout my WoC army, to keep a unified look (cavalry, characters, chariots etc), and used the metal knights for variety. I just never field them next to the palstic ones
