I doubt that there would be an alliance with all the Spaniard , but it really depends what their objective is. If they all would be happy with a podium then it's very unlikely to see anybody really trying to defeat the Sky Train. However If they're more focused on winning the TDF , then they'd would all have a common objective , and that'd mean that there will be attacking. That could be problematic to the Sky Train if Rodriguez , Valverde , Contador try to attack from far (dunno if it can be said that way in English).
There is no need from an explicit alliance , but if enough riders are focused on winning the TDF , then they will have to derail the Sky Train , and we may end up seeing Froome isolated.
After all we have seen a surprising Valverde in the Dauphine , he wasn't succesful with his attacks , but he did not wait for the sprint , at least he is showing ambitions.
Sky have a very strong team , however Contador , Valverde and Purito will have strong team too , or at least some good teammates , they could put Sky under a lot of pressure.
I doubt that it would be enough to beat Froome anyway , even on his own , he seems very hard to drop.