Great Powers: FSA, KoB, RC
Lesser Powers: Australian-Royalists, Canada, Polish-Lithuanian Commonwealth
Minor Powers: Belgium
Others: Free Hellenic Kingdom, United Kingdom of Romania, Spanish Republican Confederacy, Socialist Union of South America? Portuguese Empire
Great Powers: EotBS, PE, RoF
Lesser Powers: Chinese Federation, Kingdom of Denmark, Italian League, Ottoman Empire
Minor Powers: Scandinavia, Belgium
Others: Republic of Egypt? Republic of Chile
Great Power: CoA
Lesser Powers: Australia-Free, East India Company
Minor Powers: none currently
Others: Republic of Egypt? Socialist Union of South America?
I don't think the great powers, given their command structures and ego, should mix forces. If you want to represent combined actions take multiple forces like for a 2v2 game.
Great powers: CoA, EotBS, FSA, KoB, PE, RC, and RoF.
Lesser powers should be able to be fielded as stand alone forces in games. Lesser naval powers are the Australia-Royal, Australia-Free, Chinese, Denmark, East India, Italians, and Ottomans. Poland is a lesser power for air games. Canada is a lesser power for land games. I think lesser powers should make 2 squadrons available to their alliance great or lesser powers (one being a small) and be able to take 2 squadrons from alliance great or lesser power (one being a small).
Minor powers are factions without the ability to field stand alone forces. They should be totally available to their alliance great power and limited available (2 squadrons with one as a small) to their alliance lesser powers.
Other powers are listed in Spartan Games downloads for Alliance Nations but do not have models currently.