> You can get a ticket for driving slower than traffic, despite driving at the speed limit?

Some speed limits in California are soft (it depends which of the provisions allowing speed limits they are adopted under, or whether they are the default limits.)

It is legally possible for traffic to exceed the speed limit where those limits are soft without necessarily being illegal, unreasonable, or safe.

It may, in some circumstances, therefore, possible to impede reasonable flow of traffic while driving at the speed limit in those cases, thus breaking the law.

> Could you, in the same situation, also get a ticket for following the traffic—i.e. breaking the speed limit?

Yes, but if the circumstances were such that someone could be impeding traffic at the speed limit, it would also necessarily be true that it would possible for you to exceed the speed limit and not be breaking the law. But the burden of proof would be on you to show that the driving was safe.

Yes, and in states with a 'basic speed law', or similar, you can even earn a ticket for driving substantially below the posted speed limit. The Utah driver's handbook and license test, for instance, explicitly call out that road conditions may be such that the maximum safe speed is 0 mph.