I think it's most likely they'll get their cutie marks all at once, similar to how the mane six got their cutie marks around the same time when filly Rainbow Dash performed her first sonic rainboom. I kind of view the CMC as the Mane Six Redux as two of them are younger siblings of two of the members, and each kind of ponies are represented in both groups. I just think it's only natural that the CMC will each get their cutie marks at the same time.