Only if you stick a “na” in there as well, surely? To make it “Is there one of you who knows…” instead of “does one of you speak…”. A different construction.
Otherwise the fact it is not a “specific” noun means you use “oes” instead of “ydy”, doesn’t it?
The “'na” in there certainly complicates things ;-), but it is not compulsory, I think!
I disagree that not worrying about it is always the best way.
If “not worrying about it” includes ignoring points of grammar.
I certainly find that a knowledge of grammar improves my ability to listen out for such things- much quicker, in my experience, than simply waiting until it sinks into my subconscious. Thus, a knowledge of grammar allows me to listen out for and remember the grammar used by people I speak to. It enables me to understand and learn how it is actually used faster and better.
Just in my experience, as I say.
But also the experience of other people who are learning/have learned Welsh I know.
Not a universal truth, though, I am sure.