Could someone who knows the process explain how HS and Club soccer integrate?
1. When does the high school hiatus typically begin and end for Cal South?
One of the CIF rules is that high school sports, while in season (Fall, Winter or Spring) the player cannot participate in outside organized specific sports, except for ODP, National team activities. College ID camps are usually allowed but the interpretation is up to each region (i.e., CIF/SS, CIF/SD, CIF/LA). So the short answer is no, they do not integrate.
As for Calsouth, they have nothing to do with it. Usually, as mentioned already, Thanksgiving is the cut off but the actual HS season starts typically a week before Thanksgiving and most schools don't have any games prior to TG.
2. What time of year do High School practices typically begin and end?
Official season practice starts after the date specified by CIF each year. Many schools hold tryout in early fall or even summer and use PE as a mean to hold practices for soccer teams, but its not officially season practices. Some schools, like my kids', they don't hold a tryout until November and soccer is added onto the school schedule as the 7th period and is not a PE class, per sa. It typically depends on if the coach is a teacher that also coaches or if its a full time coach elsewhere. My kids' coaches are a local college coaches and their players so they have very limited availability in the fall.
As for the ending date, for Frosh and JV, it typically ends around Valentine's Day in February. Only Varsity that make CIF playoffs go onto further dates. The CIF finals are held in early March and it ends at Regionals, the following week so at the most, its two weeks into March. There is no "State championship" like in football, because NoCal season is in the fall and not the winter time that we use.
3. Do High School coaches typically allow Club soccer kids to skip practice on overlap days?
Do you really have to ask this??? Have you ever met a coach that accommodate other coach's priority?
Assuming that you're talking about PE class used as a soccer practice, just think about the question. Do kids get to skip class because of club soccer? And if its in season, you can practice with your club team, if they are continuing because of age at 8th-9th graders, but most playing HS do not, so, in a word "NO".
Hope these help!