I'll answer your question here, but remember that your best bet is always to read through the ROM's thread to determine if it will work on your carrier (you'll see a post from me in the grooveROM 4 thread stating that both the 6.1 and 6.5 versions worked fine for me on TELUS). You'll also learn a lot about the ROM's features and possible bugs. If you're having trouble finding specific answers, just click on "Search this Thread" and type in TELUS.
The best bet to make sure that you're starting nice and fresh is to flash the TELUS shipped ROM, which you can find
here. Make sure that you let it complete customization. Once that's done, hard reset your phone. Let customization complete again. Finally, flash the custom ROM of your choice.
I don't think that you need to worry about formatting your Internal Storage. I've had my Diamond since last August and have flashed countless ROMs and have never reformatted my Internal Storage. Your results may vary, but I would class reformatting as a last resort only kind of fix.