Mixing figures from different manufacturers is a perennial problem for wargamers; both height and heft can vary as can sculpting style. With this project I ran into all of these issues as well as the thorny issue of historical compatibility.
This picture shows how three 15 mm manufactures compare:
Essex & Feudal Castings
When I started the project my twenty
year old bases mixed both Essex and Feudal Castings figures on the same base.
As you can see the height and heft are close enough and the sculpting
styles don't clash.
But, and it's a big but, they are clearly dressed & armoured very differently. More to the point I was no longer happy mixing them.