No neither have Tobacco sponsorship on them. To be honest i have never even thought about this before, i just buy F1 cars because i like the paint schemes, unlike my BTCC collection. I have wanted the Stewart for years, like i also want a 96 Jordan, both is down to the paint schemes as much as the driver or car.
As for adding decals, i have never done that myself, not on a diecast model, but i am sure it would be easy enough, i beleive the decals are exactly the same as Airfix kits for example, put the desired decall into warm water and apply with tweezers, easy enough.
I thought about this tobacco thing for a second and just had a look at my collection, not one of them has any tobacco sponsorship on them, the Williams Renaults in particular do look 'naked' without the old Rothmans sponsorship. I feel cheated now :)