Castro would be an extremely risky VP pick for Clinton. He could easily end up being Palin 2.0. IMO he only makes sense as a Hail Mary pick to boost Latino turnout if Clinton is losing polls by 5-10% in August.
The whole idea of putting a latino on the ticket would be to boost latino mobilization, which was beyond laughably low in both 2008 and 2012. Though it wouldn't be a small feat to flip Arizona either as the icing on the cake.
Arizona and Texas are the two states where latino mobilization has been by far the lowest. Julian should be able to appeal to latino voters in those two states, though he'd be better served to spend the next 12 months learning Spanish intensely. I hope he'll get some time for that at least. At this point I fear that even Hillary knows more Spanish words than him lol.