The old term of abuse (favoured by Willie Ross I think) for the SNP was Tartan Tories, btw.
Plaid Cymru basically just runs in opposition to whatever Labour advocates, but usually by being further to the left than Labour.
Plaid are not to the left of Labour (except, perhaps, in some of their own late '90's propaganda)... they are basically in the centre of Welsh politics, with a pseudo-social democratic wing and a rather right-wing (and to put it bluntly; pseudo-fascist in some cases. Out and out fascist in the case of the late Saunders Lewis) nationalist wing.
There's a possibility that Plaid may be junior coalition partners with Labour after this year's elections, although nothing as far as that goes is certain.
In terms of Westminster politics, Plaid are all over the place and always have been. Not that there position in Westminster matters at all to anyone.