Hi again, I had a few more doubts.
Crunches : do you recommend doing them on the stability ball? or just on a mat?

Russian twist : I saw quite a few demos on the net and they all have some variation - some of them have the feet up in the air, others have the legs on the ground, some with weights, some without. Some stay up in a V-position right through, others say to come up, twist and lie back. If you could attach a link to which one you recommend - that would be great

Back extensions - would these be like the superman, but on the ground, alternate arm and leg raised?

On another forum i saw a recommendation to using the Ab roller - would you say this is worth adding to the list? If so, do I put it under the full-core options in a superset?

Thanks for all your help!

Crunches - either method works fine and you should do what you feel gives you the best contraction of the core. I do mine on a mat, but that is mostly due to not having stability balls early on in my learning.

Russian Twists - Yes, the variations on this one are numerous. I don't consider the crunch to twist a "Russian Twist" but they are a good exercise as well. Legs on the floor would be the starting position and legs raised with knees bent is the next level in difficulty. Keeping legs up and straight is taking it up another level. Start with what you can manage and then work towards the higher level in difficulty. It is the same is with the weights. Unweighted can allow for speed, but like with any strength training, some increase in difficulty should be aimed for. I don't have a "favorite" variation. Some times I go for slow controlled movement with a heavier weight, sometimes I go for lighter weight, but a more difficult leg position.

Back extensions - Yes, like the supermans.

The ab roller (or doing an ab rollout) is a good one as well. It can be difficult at first and you need to watch form and keep your back stable.