If you are trying to install a screw-on tub spout, it might not tighten up, because the SharkBite adapter will spin. You would need to use a wrench to tighten the SharkBite side, and that would be hard to do inside the spout.
Some spout basics:
Diverter vs tub only
- Diverter spout models have a pull up knob that blocks the water
flow, causing it to reverse and come out the shower head.
- Tub only are just simple spouts, filling the tub.
Spout connection types
There are three main types of how a spout connects to the pipe
1 Front threaded. The pipe traverses the length of the spout
internally, ending in a threaded fitting (usually 1/2" IPS). A front
threaded spout usually does not integrate a diverter.
2 Rear threaded. The pipe connection is made at the rear of the spout. These spouts can be either diverters or tub only.
3 Slip on. These spouts will work with just a clean, open copper pipe, as it has a set screw adapter to clamp the pipe.
Soldering to the nipple with the PEX attached is not optimum, but can be managed if you have room to tightly wrap a water saturated cotton rag around the copper and extended over the PEX portion. The water won't allow the PEX connection to go above 212 until it dries out. You will need several inches of bare copper pipe on the end to solder to. (Just TRY to a solder a pipe with any water in it.) Practice your soldering ahead of time and you will become confident of solid, leak free joints.
This thread has some good advice on soldering
This page describes tub spouts.