He doesn't need to consider it.
If we accept the most likely outcome of the election is that no one party will gain a majority, but that Ed is the leader most likely to be able to negotiate a majority in the Commons, then he could essentially dare the SNP to support any motion of no confidence and face the consequences.
The SNP's big projected gains in Scotland are largely based on their attacks on Labour for working with the Tories over the independence referendum. I would suggest that many of the Labour/SNP switchers are soft SNP - not all, but probably enough to reverse a significant number of those expected SNP gains in a subsequent election, should the SNP contribute to the downfall of a Labour government (as they did in 1979).
Even sitting on their hands and abstaining would still be painted as support for the Tories and in the general election to follow, a full frontal attack by Labour on the SNP could in fact lead to a Labour majority nationally. What would be worse is that the parliamentary election could even coincide with the 2016 Holyrood elections, endangering even the SNP's position in the Scottish Parliament.
The SNP might even struggle to maintain their grip on those voters in the face of their exclusion from the government - escaping blame for any errors, but also excluded from claiming credit for any policies that prove to be a hit with voters in Scotland. An effective Labour-led government could start clawing back those voters.
The SNP promising not to back the Tories is essentially holding themselves to ransom. They've unavoidably backed themselves into this corner, but they may find it quite restrictive.