Yes, that's the issue with assuming Alexander as the 6th SP. If he makes the big league, which he probably should, that timing of needing a spot start might not coincide with him being available. If he pitched a few innings yesterday and they find out today that tomorrow's starter can't go, then what? Bullpen game? Drag tomorrow's Toledo SP up for a spot start? I guess these kinds of situations just get dealt with on the fly.
Now, if a SP goes down for an extended period of time, I think they can build Alexander up to a SP stamina. Or maybe they have somebody like Faedo/Wentz just proving themselves in Toledo at that point, and its an easy roster exchange.
Either way, they're going to need probably around a dozen starts from Ps outside of whomever the 5 in the rotation are when they break camp.