Craigie, I have checked out the distance from Sofia to Skopje, it is only about a four hour drive. I don't think that you would need an overnight stay. We went from Sofia to Novi Sad in a mini bus, it took us seven hours to get there, and about the same back. I think it cost us around 50 euros a head, there were eight of us and the price included two drivers. I can try and find out the name of the company if you want.
Its a cultural tour, no rush.