I don't think you have much to worry about. I would try to adapt this recipe for your slow cooker.

Yay! I inspired!

- I have two huge carrots, a bunch of celery, onions, mushrooms, one tiny turnip. Can all of these guys go in there or what?

Yup. It's a good combination. I never add more than one stalk of celery, though, because then that's all you taste. Throw in a bay leaf and some peppercorns, too.

I don't know how long it'd take in a slow cooker. I usually simmer mine for an hour, or until the veggies have nothing left to give.

You can't really overcook it. The danger would be letting it reduce too far, but it shouldn't do that in a slow cooker with a tight fitting lid.

Incidentally, what I like even more than veggie stock is mushroom stock. Take a pound of 'shrooms (I just use button or crimini), cover with a gallon of water, cook. It's really yummy and makes excellent mushroom barley soup.

things not to add: anything from the brassica family. So no broccoli, cauliflower, cabbage, etc.

I also like to add garlic.

It sounds like you've got everything you need.

Break the veggies down into chunks and throw them in the pot. Add a bay leaf or two, a couple crushed cloves of garlic, and a few whole peppercorns if you've got 'em. Fill the pot about halfway with water and crank it up. I'd recommend cooking it in the crockpot overnight on low. Let it cool, strain and store.

I like freezing stock in ice cube trays and then popping the cubes in a plastic bag. Then you can use it a bit at a time without thawing and refreezing.

Okay, now it's time to go make dinner.

When you're ready to tackle chicken, here's some inspiration: Stretching a Chicken

Cool, now I can make veggie stock and then make vegan iconomy black bean soup !

I make lots of veggie stock. I tend to go with what's in the fridge, but generally I have the same stuff on hand. Mudpuppie is right about the celery. I rarely add it since I rarely eat it, but I do tend to use celery root. It adds a milder celery flavor in my view. I also always add cabbage. It makes for a nice sweetness. I like all the red vegetables so I use red cabbage, red onion, and I'll add a beet too. I'll save up mushroom stems, and broccoli stems and add those. I usually add dry shitakes too, but I call them "shy-takes" (I'm so effing clever!).

Here's a photo of a batch I made in my 5 gallon stock pot which includes red onion, ginger, parsley, dried shitake, garlic, red cabbage, red beet, carrot, broccoli, sweet potato, and other stuff that doesn't come to mind:

≡ Click to see image ≡

Good things to do with it besides soup is to cook rice in it. Lately I've been living on wehani and brown basmati rice. Use it as your liquid for braising fish. And so on. I tend to make mine pretty rich, and without salt, so it's a good thing to just heat up and have a cup of.