I initially had a G35 and got a 9mm conversion barrel for it to shoot IDPA. It worked very well. I don't think you need to worry about putting the wrong cal mag in because if you put 40 mag and 9mm barrel, it will not chamber, just jam. If you put 9mm mag and 40 barrel the 9mm round will fall right thru the 40 barrel and should not fire. Obviously tho, it is something to be avoided. I liked shooting the G35 in 9mm so much that I bought my G34. Why have one gun when you can have two?? I wanted to build a USPSA open gun and decided to use my G35. I got a threaded 40 cal barrel and put a comp on it so I could make MAJOR power factor in USPSA. Now I shoot the G34 in IDPA and the G35 in USPSA.
I shoot a G34 in IDPA, USPSA and 3 gun matches. It is the best pistol I have. It has NEVER had a malfunction other than a couple of squib loads I loaded without powder. I also have 1911's, XD, XDM and a G35. I am a Glock lover. I work in a gun store and have access to a lot of guns. Have not found anything I like as well as the G34. It has had some work on it. It was sent to Robar for a finish, not a grip reduction, and has additional cuts in the front of the slide. It also has an aluminum mag well. I don't have pics but I will try to get one as soon as I get a chance.