I can't say much about slot and tab panels, but I've found that the dimensions on my watercut wedge/flipper are at max 0.7mm greater than I expected them to be. Most of the dimensions are about 0.5mm greater.
Also, yeah for smaller holes like 6mm, I would make them a bit larger. I would maybe go 6.5mm - 6.8mm? I can't say for sure, but I made all mine 6mm in my CAD and the bolts didn't quite fit.
A 10mm hole should be fine at its normal size - is this for a bolt or a rod?

CAD is looking very nice!