That is providing I can get samples and they scale up ok.
I like your thinking, but I've had bad experiences with Kallistra. I got some vikings and Mongols to use for Warmaster and they were way too big to rank up alongside my GW, Pendraken, Magister Militum, TB Line, Copplestone, Steve Barber etc. They're 12mm or slightly larger and I'm too pernickety to tolerate mismatched troops!
Also I notice you're in West London. Are you playing regularly? Because I'm in North London and have been gameless for several years.