Would be my choice to develop a regular pairing in the heart of our defence. We know what Mads can offer and Cundy looks impressive in aerial 'duels'. Love a duel. Think he just needs regular game time and probably offers more than Kitching.
Cundys best attributes are as you say his aerial ability etc which is similar to Helik. He looks to be the best one of the lot to replicate the role Helik did for us (without being as good of course yet) so I agree I'd start with him. I would then rotate between Williams and Kitching depending on who we are playing. Val did this all the time with Solly and sibbick. If we are against a team with a pacey attack, go with Williams Cundy Andersen. If it's going to be more of a physical battle, go with Andersen Cundy Kitching.
Cundy looks so far off being a footballer so far though. Looks like he's signed purely on physical stats, tall = 70000 points.