I have a stack of MS425s with a /30 out to an external network, it works fine so long as you assign it to the SVI and not the physical member switches. Set the /30 on a VLAN interface that is different from all your switch interconnects, and put in a static route for 0.0.0.0 to the router on the other side of the /30 external link. If your Internet connection devices are set active/passive it will work on a single IP on the other side too, at least, with my firewall vendor it does. For the internal switch interconnects, the SVI should act as the internal router/gateway. Getting to the next level is then taken care of by the 0.0.0.0 static route to the device on the other side of the /30.
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