Which template to use is up to your personal taste and best integration with your Joomla template.

Each variation doesn't affect buttons width or smartphone layout, they are still the same and fixed across every template.

Templates affect only colors, styles, etc. and apply to all devices without any distinction.

There is not a specific rendering looking like a native mobile app for smartphones with big buttons, etc. This is not a mobile app, but a general purpouse web chat like the Facebook one.