As I understand it, it would not be Miliband and Sturgeon but Miliband and the leader of the SNP in Westminster - who may or may not be Salmond. Sturgeon is not even running for Westminster, remember, she is a MSP and so it's in the Scottish Parliament that she'll be spending her time.
I feel a bit sorry for the Labour Party. There are several candidates for the left wng vote, like Labour, Lib Dem, SNP, Plaid and the Greens. On the right there is only one serious challenger to the Tory vote, which is UKIP. So the left vote is fragmented and Labour suffers from that, making it unlikely that Labour will win the next election outright. But if Labour forms some kind of alliance with the SNP to get power,the likely outcome is that over the course of the following five years they will upset (to put it mildly) the English majority vote so much that Labour will not have another sniff of power for 15-20 years; and it could actually be in the SNP's interests to promote this outcome, as it has a strong chance of delivering them Scottish independence, which would mess up the Labour Party's future in England even more. So it appears to me that, unless they win an outright majority, the Labour Party is in a lot of trouble over the next few years - and, even if they do win that majority, it will only stretch these troubles out a little longer, not resolve them.