Its a matter of what you want to achieve, but IMO sticking with a single format in a session is the best way to go. If you want to play SnG, play as many tables as you want at the same time, but stick with SnG. If you want to play FR same thing.
I could go on as to why this is, but suffice it to say that when I player search and find a player doing just what you described, they are generally going out of the SnG without cashing.
Seems like a bad idea to me........