You cannot create a stereo pair using a Play:1 and One. Read more here:

https://support.sonos.com/s/article/1066?language=en_US
 

But you can use Alexa on the One to play music to the Play:1s.

? 

I already have 2 play 1's 

If I get a play 1 with Alexa will it pair with the non Alexa one to make a stereo pair? 

No. See my response above.

FYI… there is a Sonos Play:1 and a Sonos One. They are two different models. Therefore they cannot be paired together as a stereo pair.

There is no “Play:1 with Alexa”. There is a different product called a One which does support Alexa and a One SL which does not. A One and One SL can pair but neither will pair with the older Play:1. 

So if I purchase a Sonos One with Alexa, will that one new speaker control all other Play 1s, Playbars, and Subs in my system?