Tammy, you really can start with any module you want. What was the time period you studies most in the past or the most recently? Don’t do that one! Start in the next time period or start at the beginning.
Some people tweak SCM and do Ancients in fewer than 3 years by skipping a few books, doubling up some days, or even just doing history through the summers to get through more in a year. Make it work for you.
State history – do it when you want to. If you want to devote a full year to it you could, or what about studying your own state once a week in Module 5 or 6 when the US is being studied already? Or study your state over the summer one year.
As for working with multiple ages – several of us do that. I have seven children ages 11 down to 11 mos, with baby #8 on the way. The biggest advantage of putting all the family together in a history time period is we have lots of learning in common to talk about, shared memories of books, etc. It’s less work for mom! It does mean each child will start and end their school years in a different module, but that’s ok.
For the curriculum guide here is how it works. You read aloud to the entire family the books in the Family section. Generally one chapter or section per day, but up to you. Then the children in grades 1-3 read or have read to them 1 chapter/section each day in the books listed for their age group as well. The children in 4-6 have a different set of books to read independently at 1 chapter/section per day after family read aloud. These books are written more to their level than the books listed for 1-3 graders. It goes the same for 7-9th graders, they have a list for their age, as do 10-12 graders.
Do the older grades read the books listed for younger grades? No. Unless they want to do it in their free time. My 6th grader often likes to look through some of the 1-3 grade kids books, but not always. She has her own set of books in the time period to read that keep her busy.
Let me give you an example from a Module. In Module 3 this week you might read 1 chapter from Famous Men of Rome aloud to the whole family each day. Then all your younger children grade 3 and under would listen to you read a chapter in Pompeii, Buried Alive each day. They narrate to you about what was read and are done. Meanwhile your 4-6 graders are busy reading a chapter each day in Detectives in Togas after the family read aloud. They come to you when they finish and give an oral narration or do a written narration.
Hope that helps!