Not a dopey question at all. Despite having backpacked with cartridge stoves for yonks, I bought the wrong type of cartridge in the States in 04; fortunately some horsepackers who were following the same trail could use it on their multi-burner so it wasn't wasted.
Yes, gas cartridges tend to be a standard fitting so, above tale not withstanding, just about any brand's will do - as long as the thread's the same; there's more than one standard type of standard fitting!
Look for SELF-SEALING EN 417 cartridges, not the pierceable EN417s. If in doubt take your stove to the store when you buy gas and they'll check it for you.
As for recycling, some stores take back the near-empties (you'll probably never completely empty one), but it's a problem.
Coleman's website states simply "Cartridges end of life: Different for each country, check with the local authority" which ain't very good.
Campingaz's statement is more precise but no more helpful: "Cartridges end of life: Be they valve or pierceable cartridges, when they are empty, please do not leave them in the nature. You can put them in the carbage. As in metal, they will be recycled."
Suspect that applies in countries where trash is sorted for recycling.
Cadac's says: "Help and Advice info here" - yes, that's all!
Couldn't find anything on the Optimus, GoSystem or Primus sites.
A link to "fill-your-gas-cans.com" on Rekri8's page (parent company of Cadac & GoSystem) ain't active and the site doesn't seem to exist.
Unless you're using something like Coleman's Fyrestorm stoves, which invert the canisters to get as much juice as possible out of them, it's hard to completely empty a canister in normal use. I usually burn off as much gas as is absolutely possible and, shamed to admit, put it in the trash.
There, that was quite an interesting ten minutes I just spent surfing!
Off to the OutDoor show in Germany tomorrow so if I get chance I'll ask some of the above companies' representatives what their intentions are re recycling.