I am going to need a bit more info..
What OS are you running on the computer you are using for the setup and what airport utility??
When you say you disabled the Thomson wireless router.. do you mean you bridged it? Or just turned off wireless?
There are two scenarios.. each are pretty well equal..
Turn off wireless in the TG782T and run the Time Capsule (Time Machine is backup software on the computer). in bridge mode with create a wireless network.
Or bridge the TG782T and run pppoe client in the TC.. which will also create a wireless network..
Either scenario will be fine.
You do not have to reset the TC .. just set it in correct mode via the airport utility.
Even if you reset the unit.. you will not lose any data backed up on the hard disk.
An express can extend the wireless or use roaming network.. again either are possible setups.
Roaming is much faster but you need to run ethernet to the express. Otherwise setup extend wireless.. just use the instructions. http://support.apple.com/kb/HT4259
Roaming instructions are here.
http://support.apple.com/kb/HT4260