Albinosmurfs

Are you really able to clear many stacks with just a few militia

Just a few questions. 1. Unit upgrades per city like all units trained get +2 defense, does that affect units already trained there or does it simply make previously trained troops inferior. 2. I feel like I am waiting too long to build an initial troop and secondary army for my other champion for clearing. Can I get an idea of what a starting army should be comprised of and how long I should wait (I have been waiting till after I have a fortress/