I've got a Joomla website with the Momentum template installed. I would like to extend the site's functionality with K2 and community builder. Do the RT templates work with these extensions and if yes, does it work "out of the box" or is there some (or a lot) customization that needs to be done?
as with any 3rd party extension there are often customizations that are needed to give the rendering a certain look but the functionality should work. Of course when you have systems where there are various developers that create different portions of the content used there is always the potential for issues that must be addressed and that is why there are support forums to best serve as many users as possible in solving the potential issues that may arise from the wide variety of circumstances involved in the varied websites being constructed.
Both extensions work out of the box in terms of features. If you are satisfied with the default visual representation of both extensions then you won't have to do much. If you would like to have graphical elements of the template (buttons etc) also in those extensions you will have to spend time to customize them to best suit your own needs