[8525, TyTN, MDA Vario II, JasJam ROM Development] chunkymonkey:
The general answer to the first part is it doesn't really matter which provider gave you your Hermes as long as it is one. (Actually, there are some differences between Hermes models but honestly don't worry about that for now.)

Don't worry about going for a ROM that's derived from T-Mob. Basically, all the different network providers get given the latest working versions of Windows Mobile from time to time for them to either check and have a play with or to customise and release as an upgrade or in the newest issues of their phones (or as a download for current users).

My Hermes is Orange UK (i.e. SPV M3100) but I've ended up at various times having WM6 versions derived rom AT&T, T-MOb, HTC et al. The only thing that matters is that it's Windows Mobile for the Hermes.

As to going back to WM5, I think it's worth having the original ROM about. If you search around the Hermes Wiki (I'd do it but it's late and I honestly can't be bothered, soz!) you'll find a list of original ROMs available for download. Note your current version and go download it so it's there ready for you to pit back on should you ever want to. (Warranty-claim time can be a good time for that.)

I'd suggest the following as a routine, myself:

1. Note down any details you might need later such as ROM version etc. Anything that comes up briefly after soft-reset or in Device Info and About in settings and any network setting for acessing GPRS or 3G etc.

2. Download the OliPro bootloader and install. The reason for this is to protect yourself as much as possible against problems.*

3. Have a gander at all the current cooked ROMs. I'd advise initially going for a lightish version as you can always add to it with cab files for your preferred extra bits.

4. Don't go for the Kitchens yet. It's best to mess with installing ROMs before trying to make one. The people eho run the kitchen threads, and the people who wrote the tools they use, have all worked very hard on making it possible for people who don't know much to make their own version of WM6 easily. It's easy but do it later.

5. Radio is absolutely optional and in most cases not very unnecessary. Like everyone else, I always want the latest of everything just in case there's a tiny, incremental improvement but if you have no problems with reception or internet access then worry about that last, if ever. Not every radio version is 'better' than the previous. The most recent version always has the highest version number obviously, but new versions are often brought out to deal with issues in a particular geographical area (usually country-specific AND network-specific, though it may or may not be better for other users elsewhere). The latest version of radio may be better for you but it might be worse and it might stop your internet of wifi working or something. Leave it for now if you have no issues with any kind of radio-related functions.

6. Installing your apps and settings is obviously something you'll do when the phone itself is done.

*There is another BRILLIANT touch to make sure you're extra super-safe and it's particularly easy: follow this link: http://wiki.xda-developers.com/index.php?pagename=Hermes_SDCardFlashing . Basically, if you keep a copy of the a ROM image (renamed to HERIMG.nbh) on your microSD card (check the wiki item for specifics) then you can flash your phone at the touch of a button (well, two butons) from there in an emergency. even if your house loses ekectrical power during the flashing process whilst it's connectedto your PC and your Hermes won't even THINK of booting with half an OS flashed to it, you can very nearly always restore the ROM by flashing from the card.

It's all dead simple one you get into it, it's just a tad worrying before you start to take a scalpel to a new toy you just paid good money for.

You'll be fine mate. :)