Yes, you can do what you want..
Extreme upstairs is the main router??
It will work exactly as you suggest.. the Express would be better used upstairs and the extreme downstairs if you don't have much in the way of ethernet clients up there.
The express in the loft.. (I always thought of loft as in the roof area.. ??)
Can be setup bridged mode and create a wireless network.. setup roaming with the extreme.. ie same name for wireless same security setting and password.
But you might need a switch if you have multiple units for the loft.
It's is a different setup to have a bottom loft. It's a 2 bedroom, 2 bath apartment, which is all upstairs, and the loft is downstairs, its just one room, not really big but big enough to make a little "man cave" out of. When I go to the link you suggested, when I get to how to configure a roaming network, on step 3 it says "If you are configuring the primary wi-fi base station, set connection sharing to "share a public IP address."" Thats the problem, I don't have that option. The screen on the example is from a time capsule, maybe thats why it doesn't have that option but I don't. Any ideas?
The roaming setup is simple.. it makes no difference what mode the units you are using as long as both create a wireless network.. if both are bridged or both are set wrongly to DHCP and NAT or whatever.. ignore that part.
Roaming network depends on only one thing. Identical WAP setup..
So each unit has same wireless name. Same security and same wireless password.. then both will work.
But your setup requires that the unit in the "loft" man cave is something I understand.. is bridged.. as far as network is concerned. It must get IP from the unit upstairs.
This is router bridge. Not wireless bridge.