You may need to also do a "enable stacking node-address xx:xx:xx:xx:xx:xx" before the reboot to get it to come up as an active member of the stack.
Also be aware that when mixing Summit switch models in a SummitStack, all nodes will only be able to operate at the capacity of the weakest member. This means that if you have a more robust switch and stack it with a less capable switch, the more robust switch will use the same table sizes as the weaker member, etc.