Nope. For me neither is a subgroup, rather than two classifications for different media. One (slash) is the direct result of fandom ( for the /), and I understand slash is a rather wide term than signifies only a relationship, not necessarily sex. Yaoi, on the other hand, implies (usually, although I've seen it used to include more) a sexual relationship, and thus is more specific term even if it's not a sub-classification. This is because yaoi has the extra shounen-ai term. I've seen the two terms (slash and yaoi) used in the same way, but in some groups (particularly doujin, manga sharing) the term yaoi is very specific.