In very basic testing, this is what I've seemed to find:

Combo 1-10 or so gives a +10% bonus to damage. Caps at about 100% bonus past that point

Beast Dance seems to double that bonus.

Have not tested a "rhythm" leader skill.

Combo Artist, Combo Master and Combo God are yet to be tested. Initial tests were inconclusive with combo master.

I've been using level 5 blue worm (100 base attack, 1 charge) to test since they are easy to collect and won't kill things before a combo gets rolling. Also level 2 dark worms are also 100 base attack, 1 charge.

Anyone else have numbers for these skills?