Is it true that plate mount Cherry stabs will fit into holes designed for Costar ones?
No, the plate cut outs for Cherry's and Costar's stabilizers are completely different. Although a combined cutout can be easily achieved by overlapping the two;
swill's builder will graciously do the dirty work for you.
Also are they like PCB mount ones where you have to desolder the entire keyboard to fix the stab?
Not for 2u keys, it highly depends on the cutout for larger stabilizer. The wire will go underneath the plate, so make sure you have the longitudinal cutout across the switch hole, between the two stabilizers' cutout for this very purpose. Again, swill's tool is already programmed to do so.