That would work only if you pad down the signals. That mixer wants to see microphone level, and your radios (presumably?) are outputting speaker-level. And if you are going to make pads, it would be easier to just make a passive mixer with 5 resistors. As you say, you don't need volume controls on the inputs because you can control the audio levels on each of the receivers.
There are similar mixer products like that which are designed to take line-level inputs. Line-level is roughly equivalent to speaker-level (at least for voltage which is all that counts here). I would look for a line-level mixer rather than fool around padding everything down to mic-level.