The toolbar icon is a separate setting added by the Plugin to the settings menu.
None of the VR plugins should conflict with each other directly. The engine will go through each plugin at launch, based on a priority list in the config, and will pick the highest priority one it can find a runtime for.
Note that none of the plugins support every function in the XR library. If you only program and test for the SteamVR plugin and then ship with the other ones enabled, it may pick a different one on a users machine, leading to unpredictable behavior.