Welcome to the forum. The screw holes on the M18 slide take M4x0.7 screws. Which optic do you have, the Romeo 1 (R1) or Romeo 1Pro (R1P)? With you mentioning 6-48 screws it may seem you have the original R1. If this is the case, that optic won’t work well with the M18 slide for two reasons: the R1 footprint is smaller than the R1P; and the M4x0.7 screws needed will not fit thru the original R1 holes. Hope this provides some clarification and good luck with your build.