Will M2.5 screws fit the holes in the Arduino Uno?
With the screw holes for the Arduino being the larger screw holes, the 2.9mm screws would fit, but would probably need a nut and/or a washer to properly secure things. In the end if you intend to be doing more work with Arduinos and Pis in the future that require screwing things together then just go and get proper screw sizes, you won't regret it.