Can culter hammer breakers be used in a GE panel or must GE breakers be installed? I was told installing CH breakers in a GE panel is against code?

Check the instructions or datasheet for the CH breaker. Most likely not able to use in GE panels due to UL Listing. There are a few breakers that are UL Listed to work in other panels.

Cutler-Hammer does have breakers that are UL certified to fit 6 other manufacturer's panels. GE is one of them. The only way to know if you can use a different manufacturer's breaker is to do the research.

The person that told you installing a CH breaker in a GE panel was against code made a general statement that is false. Here is a link to the CH classified replacement breaker information page if he doesn't believe you. Link

Keep in mind, only the Cutler Hammer Type CL and CHQ are listed as replacement breakers allowed in other panels.

Not the standard and popular Type BR or CH.