The Tories will continue fiscal contraction because that is in the manifesto they won with. The SNP has no more or less legitimacy than other opposition parties to challenge that. The "anti-austerity coalition" would not be big enough even if Sinn Fein joined it. The term "anti austerity" is mis-used of course. The SNPs fiscal plans for the UK--such as could be gleaned from their manifesto--were quite similar to Labour's but a bit looser.
It would certainly be in Cameron's interest to give the Scottish Parliament as much freedom as possible to balance its own books, (ex public borrowing and money printing). If Scotland was free to tax banker bonuses, scrap bedroom tax, raise top rate income tax to 50% (while it probably gets abolished elsewhere), raise minimum wage, fully subsidise undergraduate tuition, social care, prescriptions etc . . . That would most definitely be a controlled experiment worth doing.
PS remember that the Tories have "everything to gain" in Scotland. It makes zero sense to write off somewhere with no electoral downside and only upside. They have a huge interest in the SNP ultimately hanging themselves.