New Cold air return at the end of duct?
That rule doesn't apply to the return air side. Return air is often overlooked and undersized because the effects of a properly installed return system seem similar to a very poorly installed system. It's great that you want to add a return! Most customers ask me to remove the big ugly register that nothing comes out of. That being said, the standards for return air are so relaxed that the logic is, as long as the furnace gets enough, it's good enough. Adding or removing anything to the duct system will change its dynamics. A return off the end of a duct plenum will draw more relative to a top cut or side cut return. It's almost impossible to predict the flow characteristics until the system is completed and even then you need a good velometer and accurate measurement methods to get a proper reading. So in general yes, outside of very critical circumstances, you can take a return off the end of the duct.

side note

You can technically take a supply run off the end of the duct but a 2" diameter hole can equal the output of a 5" take off. So if you cut a 5" side take off in the end of supply you can loose much of your static pressure and dump many hundreds of cfm into the take off. I have used this phenomenon to supply rooms that were previously neglected and/or extremely long distances away. Whenever doing this a volume damper is recommended at the take off as well as at the boot.