Looking at other nations where football is semi-pro at best and have a low population getting involved when compared to other sports, you can look at Pakistan as an example. It's basically devoid of foreigners. New Zealand is much higher up the ladder to CWLand when it comes to football so I wouldn't really compare us to them yet. Pakistan has something like 100 points in the FIFA ranking system so they'll do. I can't even find information on most lower nations' leagues.
We have 40 players outside of those that will be created just for the sim, so I think four teams is a fair amount. It worked fine with Red/Green/Black/Blue over in CW cricket, after all. I think we could have our four teams compete with clubs from various nations of our level rather than making a jump straight up to NZ or Aussie. We should be realistic enough to realise that we need to develop first before going to the big time. That's not to say we should accept that we'll be out of the big picture of the world for 50 years or anything - we can rig it so we advance faster than other teams in the world, but only development and skill-increases can do that, and it should be done over time - we shouldn't be in the World Cup finals for at least 10 seasons (even that would be practically impossible in today's world, but hey, this is CWLand
).
So I think we should get our club sides playing against clubs from the likes of Fiji or Malaysia or something like that. Not in their national league, but in a multi-national league.
So...
Four teams
National League - all four teams play each other, home and away, in traditional league style. Should last no more than six game weeks.
Multinational League - Our four teams vs. four teams from [country b] and four teams from [country c] and maybe even four teams from [country d] if you like, but I think we should stick to just three nations being involved for the sake of avoiding the simmers having to play so many games for teams that we aren't directly involved with. With three nations, you'd have twelve teams and therefore it would last 22 game weeks, assuming one game = one game week.
That means the two leagues would get covered in 28 game weeks - a reasonable season length for a developing nation. We can always expand later on if enough players show interest.
EDIT: My mathematics was flawed.