Alves is certainly a challenge for either GSP or BJ. Vicious legs and enough power to stop either George or Penn.
I do think he would be an underdog in both fights but I wouldn't be suprised if he caught either guy.
Keep in mind though neither GSP nor BJ are Josh Koscheck. Many seemed to think Fitch would school GSP in the stand-up; yet it was the complete opposite. GSP's speed was underestimated; BJ too is lightening quick.
Both GSP and BJ know about Thiago's leg kicks; they'll have a gameplan. Thiago also has to deal with what those two bring to the table. If Alves can't stop GSP's takedowns his leg kicks won't do squat.
It should be a good fight whoever Alves ends up fighting. Looking forward to it though.