Although they are only projections based upon the opinion polls, Sturgeon is being a little optimistic about the SNP's role post the election. On current returns, Labour would be one seat short of a majority and the Lib Dems, despite their collapse in the polls, would still have 23-25 MPs.
The most likely outcome, therefore, would be the disappearance of Clegg as leader (assuming he retains his seat) and his replacement by either Simon Hughes or Vince Cable (or possibly Malcolm Bruce, the current deputy), all of whom would instinctively fit more readily into a coalition with Labour.
The considerable risk of being seen by an increasingly discontented English electorate as being beholden to the SNP would, I suggest, see the most likely initial attempt at forming a coalition coming from Labour and the Lib-Dems; the Tories and UKIP would make hay on an almost daily basis along the lines of 60+ million inhabitants of England & Wales being done over to pander to the 4 million people living in Scotland were a Labour-SNP coalition to result.
If the polls are anywhere near right, of course...
The Lib Dems have their own views on Trident replacement, of course, but are less likely to be so troublesome to Miliband over this issue.
Idle speculation on my part, but it is another alternative to the confident 'we will hold the balance of power' line coming out of Ms Sturgeon at the moment.