Re planks: you're not supposed to do full planks as they can make a diastasis worse, as far as I understand. Check out pregnancyexercise.co.nz for further info.
As for pelvic floor exercises, the day after my emcs (dc1) a rude MW came in and told me I should get on with them. I did have some issues with faecal urgency after that birth but that resolved.
After I had dc2 (vbac) I couldn't find my pelvic floor for several days
. I try to do my exercises when I get into bed at night. It's easiest to do them lying down, and more challenging to do them standing up.
Squats are also great for both your pelvic floor and core.