I had recently set my popcap to 7 and forgot to reset it when I started a new fortress, and the experience was an interesting an very amusing one. So far the highest population it attained was 11, it's at 6 right now after it's entire military was destroyed in a goblin attack it just barely survived. Despite this, I have to say, Deephate is proving to be my most successful fort ever.
Having the popcap at 7 didn't prevent migration, just stifled it. Migrant came one at a time, occasionally coming in groups as large as two when the population dropped below 7. I was able to build the fort at whatever pace I desired without having to worry about attracting the entire Soapmakers Guild. Fewer dwarves made things run a lot more smoothly.
On the other hand, Goblin ambushes are actually a legitimate threat. When you have two marksdwarves consisting of the entirety of your army, losing even one can be devastating. And with such a low population, every dwarf has value (unlike a typical game when there are so many damn Lye Makers running around you don't care who lives and who dies) so casualties are a much bigger deal. Also, there's nothing quite like watching two marksdwarves frantically fire bolts like machine guns at an army of goblins that, had they not driven them off, could have easily torn them and everyone in the fort limb from limb.
The whole experience made me think that the first immigrant wave comes too early. It doesn't quite make sense that two rooms and a bin full of dolomite crowns would realistically convince people to travel great distances to go live there. In the vanilla game you can't even begin to get your shit together before the first wave arrives, let alone get a chance to be actually threatened. I think the small vulnerable band of settlers stage of the game would benefit from being expanded, and this could be done by raising the minimum architectual value requirements for the first immigration.
Thoughts?