The Blackmagic ATEMs do not have scalers on the inputs. This means that you must choose a single format for your mix. If you wish to include a source which is not at the same format as the ATEM is set to then you must use an external scaler to convert the source.
The BM ATEM 1M/E and 2M/E include a downconverter for the program output so it is possible to mix in HD and output your program to a standard definition output device. The ATEM TVS outputs only at its native resolution.