Does anyone know if it is possible to call upon a feature of another plugin when developing your own plugin? So your plugin activates another plugin and takes the output?
Yes, but normally only if that plugin has exposed an API. For the Python side, plugins can expose 'helpers' which can be consumed by other plugins - a good example of this is the PSU control plugin and the various sub-plugins that it has to control different types of power supplies.
In theory any plugin could call anyone else's HTTP API, although I would not depend on it working unless you know the plugin author will make a commitment to the API being stable, and won't randomly break on you. Likewise, you can technically call Python functions of other plugins, but these will likely change between updates of the plugin if the plugin author has not said that they are stable.