The form factor is different. You can use the old method of cutting the SIM into the exact shape of a micro SIM for your 3GS, but it has to be the exact size or it won't fit or it'll be loose.
To downgrade below iOS 6, you'll need SHSH blobs from an earlier iOS to do so. The only way to get them now is from Cydia's server, which is only available after having jailbroken and having ran Cydia at least once. If your 3GS is second-handed, you can try to see if they're available. Open up TinyUmbrella, plug your device in, and click Save SHSH.
Under the general tab on the same window, if you or the previous owner jailbroke and ran Cydia once, it should pop out a few items with the headers iPhone 3GS, and ending with the possibilities of 3.x.x, 4.x.x or 5.x.x. Those are your blobs. If they're there, then you can downgrade. Otherwise, you're stuck on iOS 6.