OP..
If you want to know which gangs occupies those areas, then google L.A. gang maps and it will give you the answer.
As far as Mexicans go, they are all over those areas and their demographics are growing as some Blacks are moving to other parts of So. Cal. Some areas still have a fairly large Black stronghold. In addition, there are a lot of Mexicans that are racist, but there are a lot that are cool with Blacks. If you carry yourself in a respectful manner and treat people right, then you will generally have no issues. But, it really depends on the person and situation.
Regarding which gangs have large memberships, there are gang message boards that covers this topic.
IMO, each area has its own politics. So, there may be things people do in one area whereas other people do something different in other areas. Then you factor in nuance which could change from person to person, group to group and area to area.
