About Elo.
http://en.wikipedia.org/wiki/Elo_rating_systemI think that ppl could play training match (match where using matchmaking you can lvup your unit, get some fame ducats etc...) at many different AP level
BUT Ranked match need to be standarized a bit. 400 or 600 or 800, two value is fine imo. 400 and 800 are two different level of play but could be set for ranked match.
This would help to understand mechanics of army at different level. (for example an army could be strong at 400 but not at 800 etc...)
Also balance is easy when you have a standard format. It also help balance mission. For example a deathmatch at 400AP could have a 12turn limit while a 800 could have 16 turn or more etc...
So every mission could be balanced for dirrerent RANKED match.
The key to have good RANK is to have good standard rules that need to focus on few AP army size.
This will improve the overall Ranked experience but still let a lot of liberty for training match.