Are there negative side effects to taking large doses of slow release vitamin C to avoid colds?

Yes but mild. Typically large doses of vitamin c are well tolerated as long as you're well hydrated - GI adverse effects like diarrhea or nausea can occur at very high doses.
Yes. Mega doses of any vitamin is not recommended. The benefits are not proven and marginal at best. A good multi vitamin should suffice.
Creates kidney stone. If the body pH is acidic you can test this by urine or saliva litmus test and this is not in the normal range between 6.8-7.4 then you need to make sure before you take vitamin c your body is in alkaline range. Otherwise you can potentially create kidney stones and calcify other areas too. Vitamin c can be used upto 1000mg per day for colds and vitamin c flush high dose done for constipation.
No. There really isn't any huge harm in taking too much vitamin C as it is water soluble, and what you don't absorb will be excreted in your urine. On the other hand, there isn't much evidence that it prevents disease.
Yes. There are negative side effects to taking too much of any vitamin. It is debatable if vitamin C supplements even work in preventing colds. Our body needs minute amounts of vitamins. Taking more vitamins does not mean your body will work better. You are better off eating and orange and washing your hands frequently.
Yes. Mega dose vitamin c can actually be dangerous. It is true that vitamin c is water soluble & you will excrete any vitamin c that you don't use. However, in order to excrete vitamin c in your urine, you must also excrete minerals as well. Therefore, excess vitamin c can lead to mineral deficiencies, especially a copper deficiency. Be very cautious in megadosing any vitamin or mineral or amino acid.
Yes. Some women who take high dose vitamin C get diarrhea from it. Otherwise it is pretty low risk.
No. High doses of vitamin C are unlikely to hurt anything since you'll just put any extra c into your urine. On the other hand, high dose c has been shown to have no effect in preventing colds. The best way to avoid colds is frequent hand washing.