I think what ever exercise you do is ok, just as long as you are eating enough calories/kilojoules to keep up with your body & BFing as well.
I have a friend here whose worked her way back from a size 14 - 16 down to her original 8 - 10 in just 12 months since having her DS. She's still BFing & I don't think there were any problems. She mostly did walking & home DVD workouts, but she looks awesome. I admire her!