The solution we have now is total fail. Why are we taking a choice away from the players? All options should be available to a player after they have weighed the pros and cons of their decision. This heavy tank limit doesn't even affect all Factions equally. Again, good idea...bad implementation. Want to get your two heavy tanks? Its going to cost you more then just resources but a good part of your pop cap as well.
Hmm, maybe instead of hitting the popcap, calling in a heavy-tank could impact your resources quite heavily, kinda like Tiger Ace. If you call in a heavy tank it could reduce your income by 15-25 % on both manpower and fuel (normal call-in costs would still apply). This would be in addition to upkeep. This would probably drain you so much it would be damn hard to keep an army going if you have 2 heavies. Which is the risk you would have to take. On top of that you could make it so that reparing tanks cost fuel if that's not enough. Then the hard limit is gone but there is a huge long term risk of fielding more than one. Resource starved while you have them and after losing them you probably don't have much money in the bank.
My only problem with limiting popcap is that it is already streched thin.