I have a relatively expensive 1/2-inch torque wrench. Not all of my sockets are 1/2" and many of them are smaller. Is there any problem with using a socket adapter, so I can use my 1/2-inch torque wrench with smaller sockets?
There is no problem using an adapter with your 1/2" torque wrench to be able to work with the size of socket you need. This will in no way affect torque settings or anything of that nature. You will most likely never have an issue with the socket itself, but more likely with the adapter failing first ... if anything were to fail at all.
You do, however, need to be aware that most smaller bolts will have a torque value which will fall below the minimum torque value the wrench can be set to. Many of these will be in the pound-inch range instead of the pound-feet range.
1/4in is the smallest size drivers normally come in.
The only problem I've found with using an adapter that converts to a size smaller than the driver is when you have to put huge loads on the wrench. Sometimes when this happens, it can break the wall of the smaller socket or break the square part of the adapter off.
Just make sure your sockets and adapter seem strong enough to handle the task at hand. For example, You probably don't want to put all your weight on a 1/4in adapter/socket with a 2ft handle.