As the furry here with probably the most experience with both furries and what they use, let me put in my two cents.
FIRST: On your "mythical" creatures, they're less 'mythical' and more like Chimeras. In general, if it's the combination of 2 or more species, it's either a Hybrid (species that work together, like wolf and jackal) or a Chimera (species that don't work together, like fish and bird). A bunch of furries see people with these chimeric fursonas and think they're either weird or trying too hard to be special and unique. Some will be vocal about it, others will be polite and at least leave judgement to themselves. Of course there's also furries who don't care what you are and consider you friend/family as a furry, and also there's other people who have fursonas like that and they're often accepting of anything as well. Don't take it personally, they don't like your fashion, not YOU, and can still be friendly at the end of things if both parties give each other a chance.
As for Second Life and IMVU. Let me put my opinions first and get them out of the way, then explain them both (as unbiased as I can).
I like Second Life, I hate IMVU. Second Life is really cool. I really enjoy going through and seeing what people have made and talking to people and just generally being social. I also enjoy being in more control over my avatar than in other "social programs" and it feels nice walking around in various bodies that make me feel like "me" in some other dimension. IMVU feels like Second Life's little cousin that got dropped on his head a few too many times and now is incapable of anything more than reading picture books and laughing at Winnie the Pooh.