Hi mommy
Vegetables fall into two groups, above ground veg such as cabbage, cauliflower, calabrese etc., and below ground such as potatoes, carrots, etc., If you are going to feed vegetables then its important to mix them up, never just feed one type. Generally speaking above ground veg will encourage loose stools and below ground will firm them up
Apart from, as you say, onions which are a definite no no, Rhubarb is best avoided and iceberg lettuce has no nutritional value but most other veggies are ok as long as too much of one type isnt given
As for quantities, a dog really has very little need for vegetables. It is a carnivore after all and the only plant based food it would eat in the wild is the stomach contents of its prey. There are many owners of very healthy dogs who only ever feed meat
Those owners who do supplement their dog's raw meat diet with veg generally add a tablespoonful or so of pureed slop to their food
If you are feeding a "complete" food already then vegetables are unnecessary nutritionally. However, things like a raw carrot do provide harmless recreational jaw exercise for a ever hungry lab! :wink:
Books on raw feeding by authors such as Ian Billinghurst and Kymythy Schultze contain information on feeding veggies to dogs
Cheers
Nigel