I am not sure about 1.st three, could well be only two... My experience with .19 up to now is that if I play with a cap of 19 the last wave (by coincidence so far always the 3rd) may already brake that cap by a few dwarves (most of the time just up to 20, but I experienced 22 at one time - but then 2 of them where kids)... and if I stick to that cap thereafter there are sometimes more migrants, or the message that my fortress hasn't attracted such... but If I set the cap down to 1 after I reached my original limit it is no more messages or migrants at all (excluding on side birth).
Of course I cannot be sure that it will truly obey the set cap, at least it had left me with my base 19 + 2 (my child cap) newborns for 5 years for now.
Edit: P.S.: And yes, on the first migrant waves I am savescumming - as I really don't like to have High Master Soaper/skilled metalsmith/skilled mason or such... for my 1st set of 20 I want consistent skills and not a bunch of weirdos - or can anyone explain to me how any person (ok, they have a much higher life time... but regardless of that) can gain superior experiences in surgery, masonry and smithing??? Nothing 'gainst Hobby Skills, but too much expertise is simply ... well, too much of it! I really would wish there would be a template for characters (like some xml sheet where we define hobby skills and the templates therefore and then the job sets (1st Field of Expertice followed by subsets thereof) and that non-embark creatures would use those templates to spend their skill points on)