I think you can also mount low but not for sure. I will ask some people I know who shoot them. I have only played around with them on some ARs. I do know there are a couple of different ones. There is one that law enforcement uses that is motion activated. It stays on all the time and when the gun moves it turns on. I actually like that one when I thought I wouldn't .
edit: checked with some people who use Romeo 5 on different guns and they advised it can be mounted low on a rail or any way any other red dot is mounted. A couple even have them on semi auto handguns.