It would make no sense to mark or consider SICO as gankers. There may be some gankers, but you’re more likely to actually find anti-gankers there, there is one in my channel even, but you may not have met him yet, he hasn’t been on much lately.
There are corps affiliated with SICO but not part of the SICO alliance, however, that are pirates and gankers, in particular Snuggle Society which @Ikarus_Cesaille mentioned, and you may want to mark those like you would any other criminal entity.
You may want to check the following thread to better understand what this is about:
EVE LinkNet community looking for corporations of all types
As you may see there, you could probably make your corp part of that community too, in order to offer SICO members interested in anti-ganking the possibility to join your AG fleets or your corp.