The number of Zigbee devices can go well above 200, depending upon the hub and how many repeaters you have. I have almost 280 devices (about 100 are switches), and have been over 300. I think you may be confusing the 200 number to the limit ST has allowed to be added via the mobile app, but that’s going away, plus there are workarounds if you have a Samsung device or an older Android device using an older version of the ST mobile app, or you’re an iPhone user.
In theory those should be repeaters if they are AC (mains) powered devices, but to be safe you should get the specs to be sure.
The number of devices you’re proposing would definitely need a repeater to ensure proper coverage and to build your mesh properly, plus ST’s hub (your coordinator) only allows for only a limited number of Zigbee devices to be directly connected as you’ve mentioned.
I have not personally used those devices, but be wary of those because they may likely use a zigbee cluster that ST does not support (especially now that Edge drivers are coming), or you may have to write your own driver. It may work just fine for basic on/off functions, but if there’s energy metering capabilities I would suspect those may use those manufacturer specific Zigbee clusters.
Also, remember you get what you pay for from Alibaba. I’ve gotten some decent stuff from that site, but I’ve been burnt and sending stuff back can be next to impossible. Also, watch out for counterfeits.