Hi!
I homeschool and I have 6 kids. They are doing 5 different grades' curriculum right now and the youngest is still nursing. I have a son doing 12th grade, and daughters doing 11th, 8th and 2nd. My youngest daughter will start Kindergarten next month. She's been doing pre-K stuff. My baby boy is 2 today and doesn't do any curriculum yet, although he participates in our group activities quite a lot and thinks he's at least 12.
I have always homeschooled, so I've done it with all sorts of different age combinations. When I first started my oldest doing pre-K stuff, his little sister sometimes listened to stories I read him and almost always did all the dancing games and activities with us. We added in one more child at a time gradually.
I've read tips and suggestions that even if you're starting with several school-age kids already you could still start lessons with just one of them for at least the first week or two and then add one more child at a time until you have everybody working at a level you're comfortable with.
Homeschooling doesn't have to be like institutional school in terms of everybody starting on the same day and going through a grade on the same "schoolyear." My oldest and third and fourth kids go from September through July each year, my second goes from May through March, and my 5th will start on a January-through-November schedule. Some grades have taken more than a year because of family circumstances and that's been fine. Occasionally someone has taken less than a year and that's fine too.
Go to the board here called A Day at Our House and look for my thread called A Day with 6 Kids to get an idea of how we do things now. (We've changed somewhat many times over the years.)
To answer your questions, it would work however it works out best for you and your family, and it's certainly possible in your situation!
Ramona