Yes, 50/3 is good to use for piecing. With whatever thread you use do a test and verify that it doesn't throw off your seam allowance. Thicker thread takes up a tiny bit more room & vice versa.
http://www.quiltingboard.com/tutoria...ce-t89997.html
Also there has been a lot of discussion lately about the use of poly thread. Evidently most people feel it's just fine to use in quilting due to improved quality.