Yeah, more then 5 wouldn’t be good IMO either, and your line-up sounds right.
I personally like a 4 player game when playing AA (any of the global versions). Any more and it seems like an eternity for your turn to come around (some lose interest). I also like having a partner to bounce stuff off of, and hopefully they will help see something I may have missed (or talk me off the ledge in some cases LOL). 2-3 player games are good if it will continue on for more then 1 day, because sometimes it is hard to get a group of people back together.
I think that AA1914 is no exception. More then 4 and someone is probably a little bored.
I would go with:
Russia, Italy, US
I also think there should be a slight power shift to Germany (from Austria), and a major tweak to naval combat and USW to make them more interesting. I’m also on board with Turks/Italians starting neutral, maybe not as restricted as you would like though.