I have two 2714b pieces that I want to put together to make a longer pole. Is sliding the technic pin of one pole over the end of another pole illegal? (If it is, how else could I make a long and thin pole like this?)
TLG has been using "bar inside Technic pin" technique, so I assume this is considered legal connection. One thing to consider though - some newer elements with pins are no longer round inside, so you cannot fit bar type of element there anymore.
One example from 42043 set (I'm sure there are more examples, but this is something I had in my head immediately):
Roof assembly has these LBG Technic, Pin 1/2 with 2L Bar Extension (Flick Missile)
Later, in Step 64, you have horns attached with half pins.