No. And I'm not saying this is particularly likely, just that it shouldn't be ruled out... but the SNP could accept a Tory government if they get significant concessions in Scotland - so more devolved power. More power over money. A changing of the funding formula so a cut in spending in England doesn't affect funding in Scotland. Etc. That wouldn't require a coalition.
Especially if the alternative is fewer concessions from a Labour government which could then try and rebuild it's support in Scotland, at the expense of the SNP.
Seems remote, but p > 0.0