Is is okay to use fertilizer/weed control in this area of California in the Fall?

No.

What about in any other part of California?

No

What about outside of California?

No.

Why? Those product waste your money and give you false hopes of being an effective fertilizer and an effective herbicide. They are not. If you want to fertilize, use a fertilizer. If you want to kill weeds, use a herbicide.

Forget about the crabgrass. It will not last into the winter. You can spot spray the dandelions with something like Weed-B-Gone. I would not use anything out of a hose end sprayer on newly seeded grass.

Where do you live in Northern California. And if you say you live south of Yuba City, I might come unglued. If you live south of YC and north of Bakersfield, you live in Central California. Sorry but that is geography. Now if you live in Central Cali, are you toward the edge or back inland? It makes a difference because of the temperature and humidity differences back toward Sac'to and then up the mountains.

What kind if grass did you seed?

How did you prepare the soil for the new seed? I'm surprised you're getting many weeds.

I am in Central California's central valley, more commonly known as "the grapevine", north of Fresno. It is dry and hot in summer, mild and wet? in winter. We don't normally have much humidity here. I planted tall fescue grass. I will try the Weed-B-Gone on the dandelions. Thanks for the tip! : )

Perfect. Yes fescue should work for you in that climate.

I wouldn't use any herbicides until that grass that you just seeded has been mowed at least 2 times. Always read all labels of any herbicide you are going to use.