1) There is more TV money involved in the Championship than the SPLThe SPL is tiered thus (IMHO);
Big Two - Premiership standard
3-7 Championship standard
7-10 League 1 standard.
Steve Thomson played for Falkirk who are definitely in the 7-10 category. Graeme Smith played for Motherwell who are in the 3-7 tier. That makes him a Championship quality player in my opinion.
2) The average gate in the Championship is about 18,000 whereas apart from the Old Firm, no team averages that in Scotland. Therefore I think you're being extremely generous to the sweaties there.
I'd say it was:
Big Two - Mid table Premiership standard at best (until there is more money up there)
3-4 Mid to lower championship standard
5-8 League 1 standard.
9-10 League 2 standard (maybe even non league in some seasons).
As for Thomson, he was so poor for us that calling him league one standard is being kind.