Slonimer and Gaon,
Eishes Ish is a greater if one commits a capital crime. But with respect to the Issur Yichud, they are the same. According to Rashi, yichud by a niddah is learned out of yichud by an eishes ish.
That Tosafos is ambiguous. I could be that Tosafos is saying is that ishto niddah is more lenient because she will become tahor shortly, but a penuyah nidda remains asur mid’oraisa just like eishes ish. You can also understand Tosafos as holding that the yichud by a nidda is only d’rabbanan. But the Tur and Shulchan Aruch pasken that it is d’oraisa.