Blue / Black Turtles Usage

Not quite sure what to do with my blue and black turtles, at the moment I use as follows:

Yellow Turtles - Shanks
Red Turtles - G3 Luffy / Arlong
Green Turtles - Lion Song / Asura

The problem I have is I'm not sure what to do with my blue / black turtles. I don't really want to use them on anything that wont be used later on in the game:

Currently have:

Blue:
Mr Prince 3*
Kuro 4* (currently used in slasher team)
Miss Valentine 4*
Plastic Surgery Sanji 4*

Black :
Buggy 3*
Don Kreig 3*

The only one I use at the moment is Kuro and I'm not sure about him later on in the game, should I just use them on G3 luffy / Shanks and just take the -exp hit, or should I be starting to level something else?

If you don't want to use turtles on those characters don't do it. It's still 15k exp for your main crew and if you have a lot you should use them on the main crew, but always keep some turtles, so you can prepare for the incoming raids

Okay thanks, just one question are any of my blue/black characters going to be any use further on in the game? Or will they all be replaced?