I like Aurifil 50wt 2-ply for piecing. The reasons are several. It is a finer weight than regular thread, so my bobbin holds much more of it making bobbin changes less frequent. The fineness of this thread makes for a very flat and accurate scant 1/4" seam for me. It is not linty, so I have to clean out the bobbin area less frequently. A spool lasts a long time.
I like the same thread for quilting on my DSM. Some quilters move to Aurifil 50wt 3-ply for this because it is stronger.
You can certainly mix threads when sewing blocks. My only caution is to make sure you are using the same weight and ply of thread. Most thread is 50wt 3-ply. They will all wash okay.