How can set Elvui so that the only thing modified are the blizzard menus (achievements, bags, etc) and not have any action bars or anything changed to default chat.
Speaking from the point of using ElvUI if you type /ec and then click on Nameplates There should be an 'Enable' box which is ticked. If you untick this it wont use the ELV ones. You can do this throughout the rest of the configuration such as the Unit Frames Tab ect..
Typing /ec brings up the ElvUI config. Every UI element has a page on that config. For the elements you want disabled, simply make sure "Enable" is unchecked at the top of the config window.