Instead of using a crimping tool to shorten your servo/ esc wires is it okay to solder directly to the rx?

Any downfalls to doing this?

Probably just unhandy to work on at times. Consider the other end, unsolder from the servo, shorten and resolder, leaving the connector, that's what some do.

This. its super easy, you just need a small solder iron tip. wires go right on the board in the servo.

I really try to keep any soldering of servo leads to a bare minimum. Heat from the soldering process causes embrittlement in the fine strand copper wires which leads to fracturing or breaking of the wire strands when movement or vibration is encountered(=loss of control/runaway). Its a pain to crimp, but it is really much better than soldering.