As indicated, strict pop cap blocks conception (existing pregnancies are still carried through if the cap is lowered), so no migrants => pop cap 1 (allows for deaths, without them being replaced by migrants), and strict pop cap = your end value.
Note, though, that you're going to have trouble with only 7 dorfs because all the factors that block kid production:
- Gender balance
- The age restriction of max 10 years difference
- Sexual orientation
- Willing to commit to marriage
- Personality traits blocking marriage
I believe you need 2 breeding pairs to create a third generation (as siblings don't marry each other).
Of course, you can get around those restrictions by accepting petitions from visiting scholars and performers, but I've found getting married pairs of those (for multi racial fortresses) or unmarried, but willing to commit to hetero marriage (i.e. produce kids) ones within the age range requires Legends Mode info (which I get from yearly exports to Legends Viewer). However, if you're trying to breed dorfs that way you could just have accepted migrants.
I found the two waves you get with dead civs to give a decent base that usually (but not always!) allows for 3 pairs of parents, and you can actually get some work done (but still slowly!) with a 12-23 dorf crew.
I don't bother to change the child limits as the rate at which couples produce kids is the limiting factor (even if you send them back to the bedroom immediately after a kid has been born) in small fortresses.
You'll also need to adjust the siege limits to get any sieges within a reasonable time.