Hello @gfmintgen
The datasheets from the manufacturers on those motors do not show the ratios though it is likely more logarithmic than linear.
Don’t take these numbers as fact but most likely if 12V=10rpm the curve would be something like this.
10V = 9rpm
8V= 7 rpm
6V = 4rpm
4V = 1rpm
3V = start of motion
If I was going to test one out I would likely start with the sparkfun as it does list the 3-12V range which gives a little more room to hit the speed you are looking for. The second gives a range of 2-7.5V though @6v is already 3 times faster than you want so it may be harder to find the speed you are looking for.
-Robert