In my opinion carrying population will add such a nice aspect to game but it should be calculated well. For carrying populations there should be a daily limit for it.
For example if fed trader can carry 500k in a day from akritori. 10 of them will completely take all population akritori have in only 4 days. in that case big nations with big amounts of players(too much ships to carry pop for their ports) will drain the low population nations blood. for prevent this there should be a limit to carry pops not count base maybe % based of total pops. and it will led bigger nations ports little bit more vulnerable(they can still cover for their lose with too much ships and players availability)
This system should work well for both less player based nations and big player based nations. For preventing to have huge powerspike for sake of big nations. Daily limit to carry pop for per person with % based seems nice in my opinion. And daily limit to a port can lose pop % based will be nice also. Hopefully it prevents to target specific ports less.
I dont know it will be good to have but one side effect(which also can add a nice brain storms to the game) is a nation can drain their pops themself to prevent lose pops to other nations.