The Xmods bearings are 6mmx3mmx2mm, I believe a mini-z uses 6mmx3mmx2.5mm, which means the smaller bearings probably work. The last number is only the width of the bearing.
I don't own a mini-z and I'm not sure if the bearings are sealed or not. As for fitting them into an Xmod, they are wider than the RS bearings, but the XMod might allow enough room to sqeeuze them into at least some spaces.