I am not an expert but GW does sell their EPIC line for EPIC Armageddon. If you wanted to use different figures to represent GW armies, that's fine. Just base them the same way. I think they are 10mm figures. Maybe 6mm?
I believe there are other fan made rule sets, such as netEPIC.
The EA rules were available online at one time from GW's specialist site but I am not sure it is still available.