Yes, they do stack, up to 90-95% if i don't remember wrong from reading some blog a long way ago, so i probably remember the numbers wrong, but they stack, and there is a limit.

And, in fact, in DMM version, seems like high end players actually build their teams based on theri abilities and synergies. As far as i've read, most powerful strategy is having a team with sun orb collecting units, and another team with solar blast dmg up units. Then somewhere else, units that increase their attack the more the solar blast is used. Alongside the 1 turn kill strategy with units that boost attack/skill chance in the first turn and a unit with an awesome area skill that can do heaps of damage on turn one.

Of course stas are important, but when comparing same star units, is better to go by the abilities and what your team needs/can do rather than 1-2K more power. And that's why units such as Rose (sadly for me, since she is my favorite) are considered pretty worthless, as any of her skills help anyone but her....and her low def along a skill that uses def hurt her even more. You could use a def pumping teammates, but that doesn't work anyway nearly as well as just increassing atk.