Well I bought some of the new Kallistra Samurai range to see if they are close enough scale wise for a Warmaster Nippon army and I think they'll do the job.

First row is Brettonian Men at Arms, Samurai, Araby Spearmen and Copplestone City Guard.
Second row is Brettonian Men at Arms, Ashigaru Bowmen, Copplestone City Archers and Araby Spearmen.
Third row is Brettonian Knights, Samurai Cavalry, Araby Knights, Copplestone Horse Archers.
The Kallistra Samurai are the same height as the Copplestone City Guard but they are broader. I deliberately used Araby figures in the picture as they are out of scale with the rest of the Warmaster range the thicker base makes them about the same height. The only real issue is with the older Warmaster figures as they are a good bit bigger. The Cavalry are less of an issue.
I had ordered Ashigaru with Yari but they sent me 2 Bowmen by mistake. Hope thats of help any questions ask away