If I used a ddt to retain nations will it develop regens properly for that nation?

Basically want to use a ddt file to retain players for all the major countries and slightly lesser ones so that there's hardly any grey players on any international teams. Basically what I want to know is if I retain a nation will it develop regens for that nation at the same rate as it would for them if the league was loaded?

So say I retained Finland and they had the following squad players:

Main - 23 players

U21 - 24 players

U19 - 22 players

If I retained Finland as opposed to loading the league would they have similar squad numbers 15yrs down the line?

Also would it be preferable to just retain the clubs in the top tier of each nation to get a realistic number of re/newgens for each nation?

It should do, yes. Although it would be preferable to retain the top tier clubs.