Considering the precedents of mabinogion, popol-vuh, and arthurian, I see no reason why Beowulf shouldn't be given its own beowulf tag at this stage. Myth.SE may decide later to not use individual tags for each work, but for now at least there's no harm.
Additionally, questions on the topic can be tagged english or anglo-saxonold-english, with the latter two being synonyms. Whichever direction we eventually go with should be consistent with a decision on the greek vs ancient-greek situation.
Basically, IMHO we don't have to limit ourselves to one tag. Someone browsing English mythology questions could very reasonably be also interested in Beowulf related questions.