nice job on the second photo =) I couldn't tell it's a mix unless you told me. I should try that technique too for night photos. Gotta buy that shutter release
A tip for fireworks is to have a black carton (I use foamcore) to cover up the lens in between the explosions (speed at 6-8 seconds). I got alot of decent results this way =) I don't know how that would work with the building lights though, Vancouver didn't have too many buildings around the firework.