In the machlokes between Abaye and Rava where an individual witness testifies that an eishes ish committed adultery, Tosafos explains that there are certain premesis that they both agree to in order to understand the exact point of argument. All agree that there is a concept of אין דבר שבערוה פחות משנים, but Abaye holds that the concept is limited to an act where the witnesses are necessary to make it happen such as gittin and kiddushin. But, when the witnesses are only there to serve the purpose of testifying what transpired, even by davar sh'berva and eid echad is believed. Tosafos also explains that all agree to the concept of שויא אנפשיה חתיכה דאיסורא so that if the "shtika" is considered admitting guilt, she would be forbidden to her husband [If not for Tosafos, I would have said that שויא אנפשיה cannot be acheived through שתיקה כהודאה since shtika k'hoda'ah is a concept of ne'emanus - trust, but will not form a neder which is the foundation of shavya anafshei]. But, Tosafos explains that Rava rejects the notion of "shtika k'hoda'ah" since there are other things that the silence can be attributed to (such as the gemara pointed out in the "tzricha" before). Rava holds that the issue at hand is not whether she is considered to admit, rather it is purely focused on whether we trust the testimony of the individual witness. Therefore, Rava disagrees with Abaye on 2 points. 1. Abaye holds that being quiet is tantamount to admitting, and Rava holds that it isn't. 2. Abaye holds that אין דבר שבערוה פחות משנים applies only to gittin and kiddushin, but Rava holds that it applies even to making a woman forbidden to her husband.
The difficulty that I have with Tosafos is that once Tosafos establishes that Rava rejects the notion of "shtika k'hoda'ah", and we know that Abaye holds shtika k'hoda'ah, that should be sufficient to answer why Abaye isn't bothered by the davar sh'berva aspect. Meaning, even though אין דבר שבערוה פחות משנים, Abaye holds that when she admits she should be assur due to shavya anafshei. Why does Tosafos have to say that Abaye holds that אין דבר שבערוה פחות משנים only applies to gitting and kiddushin, even if it applies to z'nus she should still be assur based on shtika k'hoda'ah?
Perhaps Tosafos holds that if an eid echad is not regarded in matters of erva at all, it would not even qualify as shtika k'hoda'ah. For example, we certainly wouldn't say that if someone hears a parrot say that he owes his friend money, that if he remains quiet it is shtika k'hoda'ah. Shtika k'hoda'ah only applies when it is stemming from a credible source. Therefore, Tosfaos must first reject the notion of אין דבר שבערוה פחות משנים in this context to determine that an eid echad is a credible source and only at that point would she be assur based on shtika k'hoda'ah.
Practically speaking we pasken like Rava, that we don't say shtika k'hoda'ah. BUT as Tosafos points out and Shulchan Aruch (179:9) paskens, if the husband considers the individual witness so credible that he trusts him like 2 witnesses, then she will be assur to him. Based on this we see that even Rava has limitations for when we apply אין דבר שבערוה פחות משנים. It only applies absolutely to the chalos of gittin and kiddushin, but in the realm of believing the witness, it depends on how much the husband trusts him.