Likely figure compatibilities

[Conquistador]

I have paint converted Ral Partha/Great Endeavour Brits/East Indians, Old Glory Spanish and Sailors, Tiger Miniatures Spanish, awaiting the Meridian Miniatures Steampunk Infantry, and going to order Gringos Spanish figures for Venus actions.  I plan to run against GW Lizardmen (yeah I bought them off Ebay - not everything GW does sucks,) and Star Wars aliens initially.

What concerns about scale/size compatibility do you think I might encounter with that mix?

Gracias,

Glenn