er...a demo charge from a different squad I hope? The unit that gets out of a blown up transport isn't the same unit you chose to shoot at so you don't continue shooting at them with your squad upon a successful melta blast :/I have always managed to use them to pretty great effect popping transports with the melta's and then tossing the demo charge on the squad that comes out of the transport.
I'm liking the lists you've come up with, specially subtly modified one that donkey555 posted up. Something I've found with my own Guard army is that when I mix blobs and tanks I lose but when I go full blobs or full tanks I win. That isn't to say that my mixed armies haven't been successful in their own ways, but when I focus any of my armies on one theme they work better as a team. Have you considered lists that focus solely on infantry or tanks (collection permitting, of course)?