Am I able to use a part that resembles the couplers, but they are much longer. The part is a long, threaded metal rod. Is this legal, or must it still be a VEX part?

If you want an official answer from the GDC, you can ask here:

However, you can usually find an answer in the game manual. In this case, R5 states that robots can only be built from official VEX parts:

Rule R7 lists all of the additional non-vex parts you are allowed to use.

Based on this, I don’t think that couplers would be allowed. However, you might (probably not) be able to interpret them as being part of the allowed hardware, so you might want to ask in the official Q&A just to be sure.