I think the pop cap if there is any should be optional. Pop caps get in the way of strategy, especially the redicilous low ones. Just take a look at Age of Empires. You could have only 50 units present. Age of Empires 3 was also hilarious. The few hundred soldiers don't come anywhere near the thousands of soldiers present at Napoleonic battlefields. And if you played the Empire Earth 2 demo you know things can get even worse. That game has a normal pop cap for the units, but it also limits the amount of each building you can build in each territory. That's just hilarious. /apart from that, a good fleet has variation and numbers, not just 1 of each ship or 5 of 1 ship.
Now like I said before I DO think there should be some way of limiting the amount of units present, however I think it must always be a soft limit, however there should never be a limit that you can't pass no matter what. Therefore I think the cost increase of units would be much better. It also provides you with a good reason to build a balanced fleet. Even so, the more I think about it the more I'm beginning to like the idea. The same goes for the increase value of each unittype. I can really imagine a no increase for the basic stuff such as fighters, a larger increase for specialised ships (resulting in a small increase for specialist fighters like the A-Wing, and maybe a very small increase for X-Wings as well, just to make you consider the Z-95, but that can also be arranged by normal pricing) and a normal increase for basic capital ships (all compared to other ships of the same class). The same ratios could apply for ground forces.
In addition the models should not be too detailed. The attraction of an RTS isn't in it's details, it's in it's battles, and the bigger the battles the better.