Yes it is. MSTP is designed to be backwards compatible with RSTP (and the older legacy STP). BPDUs carry the version they operate at, namely MSTP, RSTP or STP.
If an MSTP bridge receives RSPT BPDUs then the port will migrate to RSTP mode. Same if it receives legacy STP BPDUs. You should be able to see this on the port config.
RSTP makes STP faster.
MSTP takes RSTP and makes it work over more than 1 (MSTI) instance, whereby different VLANs can be assigned to different MSTI; the idea is to block some links in the topology for some VLANs and other links for other VLANs.
In practice MSTP MSTI have to be configured, and will only work if all MSTP bridges are in the same MST Region, which also needs to be configured and is also based on the exact assignment of all possible 4096 VLAN ids to the same MSTI instances. In practice nobody bothers configuring MST Regions and even if some do create MSTI instances, in the absence of a common MST Region, these will all get collapsed into the MSTP CIST topology, which is the same topology that STP or RSTP would use. Even if you do get MST Region and MSTI up and runnnig, an RSTP/STP bridge will only participate in the base MSTP CIST topology.
