I don’t think there is any system which would allow you to plug-in “arbitrary” zigbee devices, because the zigbee standard allows for multiple different “profiles,” and not all profiles are interoperable. In fact some profiles belong to a single manufacturer and are not shared with others.
SmartThings uses the Zigbee Home Automation (ZHA) profile, and can also work with some ZLL profile Devices.
It does not support the Zigbee Green Power profile.
The official SmartThings/Hue integration only works with hue bulbs and light strips attached to a Hue bridge. Not with the tap switch or dimmer switch or motion sensor.
There is one community member who has developed some code to allow other devices which are connected to the hue bridge, specifically including the Phillips Tap, to also be used with SmartThings when they are connected to the hue bridge, but set up is more complicated than with ZHA Devices and it’s still somewhat experimental.
As far as the Xiaomi Devices, they appear to be quite well engineered and are very inexpensive, but they are designed to work with their own Gateway and have not been certified to other zigbee profiles. There are some community members using them with SmartThings, but they can be quite difficult to pair initially and after that there can also be some problems. 
In particular, because they are not certified, it has sometimes happened that SmartThings will make a change to the cloud and then the xiaomi devices will all stop working for a while. So if you buy them you run the risk that they might just stop working some day. You can talk to people who are using them with SmartThings in the following thread: