I don't think shorts have much to do with sore sit bones - they relieve chafing, but were never really meant to be heavily padded (that's a recent innovation and most people find very padded shorts to be more uncomfortable than lightly padded ones anyway) Your sit bones are probably sore because they aren't used to the position and/or the saddle.

I'll do short rides in street clothes, though I tend to avoid jeans because of the big seam (I don't know how we did it, but my mom and I used to tour in regular clothes... the idea gives me the heebie jeebies and brings up images of pain these days ). I commute in my kit, even though its fairly short, the commute that is, - I think 5 or 6 miles each way. We have a locker room and showers, so I just do all that when I get there, so I don't have to be concerned about being sweaty/dirty.