"NEMA 1-15 & 14-50 in same faceplate
That's not a problem.  Anything large like this gets put together like Lego.  

First you get a multiple gang box.  

Then you fit a duplex NEMA 5-15 yoke in one position and the 14-50 yoke in the other position.   

Then you go get a faceplate that has openings for a duplex receptacle and also a large opening for a large socket.  These faceplates are the key to it, they are readily available at a real electrical supply house, your local big-box not so much.   

All the large connectors - NEMA anything-30 or -50 - are of a standard size and use the same faceplate.  

You may need to work backwards, find the faceplate first, figure out how many gang box you need, then position the receptacles to match the faceplate.