Wow, I'm surprised that you actually use a Nexus phone tho (or is it a Moto? basing it off the screenshot here) and know about Project Fi. Usually it's only the techies like me that know about both.
Anyways, I believe that before you switch to Android Messages, you have to disable Project Fi with Hangouts. This might have been why you weren't getting messages before without Hangouts. Basically you have to "uncheck" the box in Hangouts settings. Here's an snip of the instructions.
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I don’t know why it does what it does. ¯\_(ツ)_/¯
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I would say that too, to the peson I just made taste motor oil!
Did you know old motor oil can be used to fertalize you lawn