When you setup a roaming network with the ethernet backbone as you have done.. you do not extend the wireless.
So you create a wireless network on the express with the exact same name and security functions as the extreme.
See http://www.apple.com/au/support/airport/basestations/
In particular the roaming one.
http://support.apple.com/kb/HT4260
The setup may be a little out of date with some of this but it should still be easy enough to follow.
IMHO the automated channel setup does not always pick the best channels.. so the one thing I would recommend if you have issues is to fix channels yourself.. at 2.4ghz there are only three that do not overlap.. 1, 6, 11 and you should use as widely spaced as you can.
For 5ghz it is much less of a problem. Partly because the range is so poor the interference is not so great and because there are many more channels.
From what I hear.. AC ones are fixed on 5ghz. But I need to confirm that.. so adjusting 5ghz on AC extreme is pointless but fine for the express.
Meaning, if I go from one floor to the next with a laptop the speed drops significantly and I have to turn off wireless on the laptop then turn it back on for it to connect to the closest AirPort. Any thoughts?
Roaming is still not 100%.. some clients work great.. others less so.. get your system setup properly and see how it goes.