Yeah, to level up skill, you just need to train that card with any card have same skill (both Stage Appeal and Unit Skill), member in same group have same Unit Skill, but N card have different Unit Skill than R/SR/SSR.
The card only need one same skill (Stage Appeal or Unit Skill) to raise both, but if they have same both skill, you have chance to raise 2 level up (it mean total is 4). The higher level skill have lower chance to level up.
So you can use N card to train skill of R card and even some SR card (same Stage Appeal)
You can use R card to train skill of any SR/SSR card (same Unit Skill)
Some cards not have Stage Appeal, so is hard to train their skill (I'm even feel the success rate of those cards is even lower than other cards).
Break Limit only help to increasing the level limit. Of course if you want a strong team, just use those copy of SR cards until you get extra SR to replace in your team. If I have 5 Momo-chan SR cards (day dreaming) then I'll never Break Limit her (unfortunately I'm not even have one).