Answering your questions:
In most cases switching stack modes will delete the startup configuration file of the unit. Configuration will be retained only in the following cases:
SG500X – switching from Basic Hybrid to Advance Hybrid and unit ID is set to 1
Sx500 - switching from standalone to Native or Basic Hybrid and unit ID is set to 1
Sx500 - switching from Native to Basic Hybrid and unit ID is set to 1
2. stacking mode hybrid advanced vs basic
native mode supports only single type of units;
hybrid mode consists of a mixture of unit types when both Sx500 and SG500X uits are placed together in the same stack
when choosing basic hybrid both Sx500 and SG500X units are used in the stack with Sx500 feature set, this means no support across the stack for VRRP, RIP. In this mode any unit type can take the master/backup-master roles. Note that in this mode, the GUI display the pages of Sx500, even if the stack master is SG500X, since the feature set is of Sx500
when choosing advanced hybrid both Sx500 and SG500X units are used in stack with SG500 feature set with the addition of VRRP, RIP and key-management. In this mode the Sx500 units can be a slave only, therefore up to 6 Sx500 units can be stacked.
In Hybrid mode – All units must use the DA 1G/5G ports as stacking ports.
I hope this helps.