We can stay at any of these cities and then take a side trip to another 1 or 2 - we want to be gone for 7-9 days. Is this possibel? If so which city would be the most centrally located to vist other places?
I would drop Venice as that is out of the way from the other cities. I would stay a minimum of three days in the other cities. Does the 7-9 days include the day you land and the day you depart? If it does I would only pick two of the cities you listed.
Venice seems kinda out of the way...is there any chance you meant Vienna? If you did mean Venice, my advice would be to drop it. It is not placed well to put it into a nice loop with a one week trip. So in which case, I would fly into Budapest - 3 nights, Prague - 3 nights, Munich - 2 or 3 depending on what remains. Munich is fine in 2 days. Good side trips from Prague are Czechney Krumlov. And from Munich, Fussen where King Ludwigs Castle is. Overall Munich is probably the most central for jumping off to other day trip places. However, in my opinion, I wouldn't give Munich more time because of that. There will be so much with Prague and Budpest that you won't feel slighted if you give those two ample time.