Hello Zacarias
MST can be considered framework that allows for the organization of multiple instances of STP to function in an efficient manner by grouping them into instances. The actual STP mechanism that runs within each instance is Rapid Spanning Tree (RSTP). This is the case for both the IST, as well as all MST instances.
I assume you mean root bridge election. Remember that the bridge ID for any STP configuration is composed of the priority and the MAC address. But the priority itself is composed of a value between 0 and 61440 but in multiples of 4096. Why in multiples of 4096? Because PVST+ uses the system ID extension as part of the bridge ID. This is just the VLAN number. So if you’re using VLAN 24, and a priority of 4096, then the bridge ID is 4096+24 = 5020 plus the MAC address. For VLAN 25 PVST+ will use an ID of 5021 plus MAC address. So again, why 4096? because VLAN numbers range from 0 to 4095, and there must be enough “room” between priorities to fit in all possible system IDs or VLAN numbers.
So when MST listens to PVST+ BPDUs it receives on its boundary interface, it checks to see if they are superior compared to the VLAN 1 BPDU from PVST+. If they are superior, then the interface will be blocked and will show a broken state.
For this reason, PVST+ BPDUs must have a priority that is at least 4096 lower, in order to ensure that any VLAN value (between 1 and 4096) will not make the PVST+ BPDU superior, and result in a broken state.
Regarding the compatibility matrix, what type of STP are compatible and how are the interactions?
The following lesson is helpful for examining the interoperability between MST and PVST
You can find additional information about various types of STP at the following Cisco documentation.
I hope this has been helpful!
Laz