I moved the stock derailer to the rear (Shimano Acera) and used an old Suntour Honour as an idler... because it is an open cage rear d I can remove the chain and run without it and found that the bike runs more smoothly with than without and keeps the chain up a little higher.
It adds a little chain tension which also aids in shifting but does make a little bit of noise... it is lined up with the middle of what is an 8 speed cassette.