Could you move the gravel and filter (or at least the filter sponges, if it's a built in filter) from the 20g to the 10g while you sort the 20g out? That way you'd keep most of the good bacteria with the fish.
That said, if you have the rocks and stuff in advance, it won't take too long to re-aquascape the tank. Maybe if you did a partial water change at the same time, you could leave the fish in while you add your new rocks and move things in the tank around. I get the impression that folk manage to change things around in their tanks quite a bit without taking the fish out, but I'm afraid I don't know which would stress the fish less.
I don't have the experience to advise you about the chemicals you have, although I know that many people on the forum are reticent to use any of the Ammonia locking chemicals in their tanks because I believe it can interfere with the bacteria colony and might give false water test results. Have you used the MaraCare already in the 20g?
I'm sure someone more knowledgeable will post soon and help you out.