A small red dot like shown in the rear dovetail? That's OK. The sight is light enough to not be a big issue with the recoil operation. But put a rail and a big heavy red dot or handgun scope directly onto the slide and the whole mess becomes way too heavy for the recoil and cycling to work correctly. Lots of jams will be the result.
That's why all the IPSC and USPSA race guns use a mount that extends down onto the frame which doesn't move. If the sight you want to use requires a pictatinny rail and it's of much of any size and weight at all over what these small red dots weigh then you're in for problems. Instead you need the same mount as the race guns use.