When you ARE able to access from underneath a cleat, or any other deck fitting, such as a support post for a T-top, seat, etc.....you really should use a backer plate to help distribute the load over a larger area.
But, it's not always possible to access underneath, and that's a real PITA. In cases like this, you should always try to position the fitting where your mounting holes are going into more than just some sheet metal, try to find a rib or other structural member where you can at least try to get one of your fasteners into some material.
As for the rivet nut kit, I recently acquired one myself, from the Harbor Freight store up the road (we have one about 30 miles from here) I went back on my jetboat, and all the hatch panels that were riveted down, now have rivet nuts, so all I need to do to remove a panel is use a screwdriver to remove a 10-24 ss screw. Did the same thing to my wire harness cover, which was also riveted down.
All these years of metalworking, I can't believe I didn't own one of these things before now! Wish I would have had it when I first built the boat, it would have saved me a lot of BS with trying to secure hatch covers!
Anyhow, I promptly used up the rivet nuts supplied with the kit, so I bought more from McMaster-Carr. While I haven't done any strength testing with these, to see what kind of force you have to apply to rip one out, I can say that in theory, the tighter you turn the fastener, the more the backside of the rivet nut should flare out, making it hold tighter than what you can set with the tool itself.
I would DEFINITELY use the 1/4x20 size rivet nuts for attaching cleats, 10-24 and smaller is going to be way too skimpy.