I understand that in the future we'll be able to send all the gypsies Cravers onto the frontline or somewhere far to work on the chemical factories but how about the possibility to enroll specific layers of population into manpower? 

It could be done the same way as a Religios law of "Race Purity" but for Militarists. Or it even could be more specific as Militarist Dictatorship law with due approval debuffs.

Since all of the races both minor and major have specific bonuses to working under specific conditions it would be a good option of getting rid of someone you don't like whether on political motives or any others. 

Unless I'm mistaken, I think manpower and population are technically different resources. A certain amount of food from your systems goes towards manpower generation, which slows down population growth.

For Vodyani it takes 1population from every ark into Manpower when you enroll.