I have a pair of Rondine A, which I assumed is a later model than your Rondine Gara fins. The way you described the mechanism on your fins, I do not think it to be likely that you can use these straps.
The spring straps, in the simplest term, is a spring attached with two quick-release buckles. In order for these to work on your fins, you will need to have the matching buckles (female) attached to your fins. The Rondine A has these female buckles attached with a limited rotation angle on the fins themselves. In your case, I don't think your fins come with these buckles.
If you are really creative, and are handy with tools, I believe you can get the matching female buckles (Cressi BZ 170015 Fin Buckle Pair), provided your fins have the hard plastic holes that will fit the buckle insertion.
Or, you might have better luck using one of those universal version of spring straps (IST Universal clamp stainless steel coil spring fin strap with style), and work your way through it. (Assuming you have metal stud pairs on your fins)
Obviously, you could always buy a new pair of fins and go from there... :)
For sizing, I have a pair of small/medium fins, so I got the small/medium straps. I would think a medium/large would probably fit your size, but that's just my guess. And keep in mind that the springs will loosen over time, too, as is my case. They work relatively well, though, as I saved a lot of time donning-doffing.
Hope this helps.
Tracy M Very helpful Thanks!