You would definitely want to check the connector manufacturers literature.
I know that the Arlington #814, which is a basic two screw 3/8"[1/2"KO] NM connector I often use, is listed for a bunch of different cable combinations, some of which I would not usually consider using, like two 14/2 and one 12/3 in the same connector. I stock some other two-screw connectors made by Topaz that are rated basically the same.
Even the Arlington "black button" plastic 3/8" NM connectors can be used with two 14/3 or two 12/3 cables, and both the T&B and Arlington's plastic push in "wedge lock" style connectors are rated for many different cable combinations as well. These wedge lock connectors are also rated for use with flexible cords within a certain range of diameter.
It gets a little ridiculous with some of the larger size plastic NM connectors, like the 1" and 1-1/4" where they are rated for use with up to nine 14/2 or ten 12/2 as well as several other cable combinations.
