Though I do wonder what happens if you do this after the fort has exceeded the new cap. Does it just let the surplus die off naturally or do the last additions become sudden strangers?
Far as I can recall, it just stops sending migrants after it realizes its over the population cap. You can and will potentially get a nudge over that limit.
Additionally, far as I know the "strict" population cap only adds births to the values considered, and stops any further conception in the same way that the baby/child cap would. It mostly seems to exist to the instance where being at your population cap without disabling babies/children entirely leads to a steady increase in population anyway.
So in that case, it's probably safer to keep the pop cap artificially low with the understanding that it will rise slowly due to reproduction.
If I set the cap to less than the min for sieges or whatnot, would natural reproduction by itself cause sieges and whatnot to begin to occur? Or would the hard number indicate to the game that they should never happen?