The MS425 range have a slightly different stacking approach to the other Meraki switches as I’m sure you’re aware. As they use the front ports they support a greater range of SFP+/QSFP modules for stacking, and so you can use, for example, 10GBase-SR modules (or 40Gs modules) and create a stack over optical fibre distances, if you want to.
Once you have the Flexible Stacking on the MS425 configured, it operates exactly the same as any other stacked switches in the Meraki range (e.g. MS350).