An electrician told us that our circuit breakers (ITE) doesn't match
with our service panel (Westinghouse). He said it's a violation of
code. He recommended that we should have them replaced.

I was trying to get some information on this. But couldn't find much
from the web, e.g.,

1) Is the mistmatch a serious problem?
2) What brand circuit breaker can match what brand panel?

Any information on this is appreciated.