As jerrm noted, I meant that you can flash ITead’s Sonoff branded “ZBDongle-E” and “ZBDongle-P” with Zigbee Router firmware as either of those can make great Zigbee Router devices when flashed with alternative firmware if then used in a USB charger for power so can be placed anywhere as a stand-alone device. The benefit is that these have an excellent modern radio and external antenna + get the latest cutting-edge firmware so will work better as dedicated Zigbee Router devices than practically any other commercial dedicated Zigbee signal repeater device products out there. Other than having to reflash firmware and not coming with a power supply the downside is that they are ugly and large so not as easy to hide away out of sight.
IKEA Trådfri Signal Repeater already ships with Zigbee Router firmware by default so it does not need to be reflashed but it is still a good idea to update its firmware via OTA (Over-The-Air) after it being paired/joined to Zigbee solution. The benefits of the IKEA Trådfri Signal Repeater as a product are that its default firmware it ships with works out-of-the-box with all Zigbee solutions and it comes with a power-supply. The downside is that it uses older firmware (even after updating firmware via OTA) and it does not have as strong radio + internal antenna so gets weaker reception. Yet is does have a better radio and antenna than most other dedicated Zigbee signal repeater device products that are sold with Zigbee Router firmware by default.
Anyway, if starting a new Zigbee network from scratch then I generally recommend buying and connecting at least three mains-powered dedicated Zigbee Router devices at the beginning before adding any other devices to let them create a stable mesh network which will act as the backbone network that is always on and always available. If they already have the correct firmware then simply join/pair them to get good coverage and range, add additional mains-powered Zigbee devices, then wait around 24-hours before adding any battery powered Zigbee devices.
Not in ZHA. Not in Phoscon/deCONZ or Zigbee2MQTT either, though they have the benefit of being external stand-alone Zigbee hubs/gateways so could technically allow you to install several instances of Phoscon/deCONZ and Zigbee2MQTT as long as each instance has its own Zigbee Coordinator adapter. However, you can run ZHA, Phoscon/deCONZ, and Zigbee2MQTT separately and run them all side-by-side at the same time as long as they all have their own dedicated Zigbee Coordinator adapter, just as you can also have a Philips Hue Bridge, an IKEA Trådfri Gateway, and a Samsung SmartThings Hub in the same house too.
They will all work separately by themselves as there is no knowledge or communication with each other at the Zigbee layers as they will all be on their own isolated private network that uses unique encryption using a random encryption key. They are fully isolated from each other and their signals will only slightly interfere if using the same Zigbee channels, (still, Zigbee signals are very weak so Wi-Fi much stronger signals will always interfere a lot more). Any interaction between them has to be done via integrations at the application layer, i.e. in Home Assistant.
Also, be aware that Zigbee Coordinator adapters can only be connected to one Zigbee solution, and also that a Zigbee device can only be connected to one Zigbee Coordinator. So you can never connect a Zigbee Coordinator adapter to two Zigbee solutions, nor connect a Zigbee Router for Zigbee End Device to two Zigbee solutions as they will be using different Zigbee Coordinator adapters. Rules are as such, one Zigbee Coordinator per Zigbee solution, one Zigbee network per Zigbee Coordinator, and one Zigbee Coordinator for each device.
This is really all due to Zigbee specification limitations so maybe best if you read up on the Zigbee protocol technology/architecture, suggest start with wikipedia → https://en.wikipedia.org/wiki/Zigbee