Congrats and welcome. I did the exact same thing with the Dowling method and am now 4 chapters into LL. I just stuck with memorizing the noun declensions and verb conjugations, though.
In answer to your question though, check out Naxos and Lesbos using an online dictionary tool (such as http://athirdway.com/glossa/
). Lesbos and Naxos both look to be 2nd declension nouns that are actually feminine.
I think, though, that you might be thinking that Naxos and Lesbos have to be feminine to agree with 'insulae graecae'. This is not the case. (I apologize if I am reading your post incorrectly). For instance: Roma est oppidum magnum. 'Roma' is feminine, but 'oppidum' is neuter.
As always, if I am talking out of my hind end, please feel free to correct me.