I like to put exhausts together using Easy-Seal SS band-type clamps, similar to the VW type pictured above. They are very expensive but heavy duty, and reuseable, come in sizes ranging from 2" all the way up to 6", and make mods to the exhaust a breeze. Available from good auto parts stores or heavy truck suppliers. Most heavy truck exhausts are put together with these.
There's also V-bands and clamps. You weld the V-bands on to the pipe ends and then the clamp holds them all together...Easily removable / re-attachable!
Don't think so - you just weld the ring on to the end of the pipe.