2. I think that one should work. You absolutely don't need to use the ones in my shop, any beacon using Bluetooth Low Energy will work fine. The thing with the ones you linked to, is that if you pair them with your phone and use their app, then they will stop broadcasting advertisements and the Happy Bubbles detectors will not be able to see them because they're connected to the phone. So as long as you don't use their apps, you're fine.
The other thing though is that beacons work best for presence detection when they advertise every 500ms or sooner. It depends on your use-case, but that's the best for most people. With beacons like the ones I sell, you can adjust that advertisement interval how you want, to decide between better latency or better battery life. With the one you linked to, I'm not sure that kind of fine-tuning is possible.
I hope this helps!