Both of those are good exercises. Make sure (and Pilates will tell you this) to keep the stomach in when you do the exercise. That protects the back.
The most simple pilates stretch/abs I do after riding and it helps. It sounds stupid, but you lie on your back with hands over your head and legs should width apart, toes NOT pointed but feet flexed. Sucking stomach in, pull yourself seated slowly, pause for a half-second on top and, keeping feet flexed, gently go as far forward as you can feeling the hamstring stretch, hold a second and slowly go all the way down, sucking stomach in. The gentle rollup of the back through the situp (with the feet in that position) gives a core ab workout (nothing too hard after a ride) and moves through to stretch the legs. I find that it stretches the legs, back, etc. Typically, as with all Pilates exercises, you only to 5 or 6 of them. Sounds stupid, but they add up to be very effective!