The general idea of the fan is (just) to keep an even temperature in all the areas of the incubator... Read upper and lower areas, corners, etc... So, you don't need much air displacement in a small incubator... I believe the 1200-1500rpm (are these the more common ones?) is more than enough.
You can see my DIY foam bator (link below in my signature), for one more example... It's an 80mm, blowing up (not at the eggs), honestly don't know the rpm's, running on 10V (the repurposed transformer I had at hand)... I've had 100% successful hatches (on fertile eggs) with it.
In my experience, it's easier to keep a stable temperature and humidity with a fan... And faster to reach the desired levels after you open the incubator, for example.