As per subject, will I have any connectivity issues?
The ZBT-1 doesn‘t work in mixed Mode. You can use it only for Zigbee of for Matter alone, but not both together at the same time. As far as I know, the development for the dual stack firmware stopped some time ago because of some technical issues. Anyway it is a good stick for Zigbee or Thread usage and also recommended for thismuse case.
You can read about this here:
Multiprotocol will remain an experimental feature and is not recommended for use.
A very good alternative is the „Zigbee 3.0 SMLIGHT SLZB-07 USB Adapter Smallest Thread/Matter Works With Zigbee2MQTT, Home Assistant, ZHA“ which is also configurable for Zigbee or Thread by installing the corresponding firmware.
Does it support mixed mode though?
They really need to make this more obvious, this issue comes up way too often!
I don‘t know. There is a Multi PAN firmware available, but it is not recommended to use that one.