Unless the stick supports Texas Instruments Z-Stack, no. Zigbee2mqtt is based on zigbee-shepherd, zigbee-shepherd is based on Texas Instruments Z-Stack which means that only Z-Stack based device can be supported.
Definitely not! The default zigbee2mqtt firmware indeed supports 15 devices connected directly to the coordinator. However, by having routers in your network the network size can be extended. Probably all AC powered devices e.g. bulbs serve as a router, you can even use another CC2530/CC2531 as a router (which has a limit of 21 devices). Various zigbee2mqtt users have reported that networks of 40+ nodes work without problems. Don't want to buy a router? No problem! In that case an alternative firmware can be used which supported 44 devices directly connected to the coordinator.
When using the default coordinator firmware + 2 CC2531 routers your device limit will be:
- Coordinator: 15 - 2 routers = 13
- Router 1: 21
- Router 2: 21
- Device limit of 55 devices