If the Tories are the largest party it means nothing. They'd be well short of the number of seats (at this stage at least) that could hope to make life difficult for a determined coalition. And dont rule out some wording from Labour and SNP that gets around what they've said already on the issue of coalition. They've merely said there'd be no formal arrangement, which leaves a very wide road through which other pacts can be driven through and without breaking the promises of what they've said so far.
It's called supply and confidence - which is almost certain to happen.
But if Labour cut into the SNP vote and the Tories end up with more seats than Labour because of a lack of confidence that Labour can keep them out on their own down south, then the Tories could go for a minority government.