What other parties? Even with the DUP and UKIP supporting them the "anti-Tory bloc" of Labour, SNP, SDLP, PC and Greens would have more MPs. And that's even if the lib dems agree to another coalition, when there's considerable pressure within the party to go with Labour instead, and when there's potentially deal breaking issues around things like the EU referendum.
Yeah, the Cons best chance is a LD coalition and despite what Clegg says, most LD MP's absolutely do not want to get in bed with the Tories again. I Clegg does try to foam a coalition I can imagine an internal revolt, or many LD MP's abstaining from voting which would result in a Labour government.
conversely, only a small chance the SNP wont vote in confidence for Labour. The Lib dems will at 25-28 seats, the SNP at 49-54 giving the anti-tory bloc a bigger share. furthermore the lib dems would never work with the DUP or UKIP, while the SNP wont mind working with the Greens, PC, SDLP etc. Finally, there is a much bigger chance that the Lib Dems would support labour, than the SNP will support the Tories