You're very welcome. Basically you don't need to worry about overdosing; they might get 'the runs', is about it. If a normal daily dose is 30 mgs, for example, giving 1000 mgs is still not going to be an issue.
Mega doses are not often required to obtain detoxification, just a higher dose than normal will do it, but I tend to overdo it when I know an animal has had something very toxic, just to be sure. I've never seen adverse reactions to vit C. But I am fussy about whether or not it has additives, generally. The lactose in some vit C's is no good for ruminants or even the average animal.
If you, for example, crushed up a vit C 1000 mg tablet for every 2 chooks if you knew they'd eaten something toxic, that could be sufficient. I get ascorbic acid powder to add straight to their food though, easier than crushing tablets. Helpful precaution when you have new chooks or other animals exploring a landscape containing deadly ornamental plants or insects they've never seen before.
Provided the 'patient' does not have advanced renal failure or one of those rare genetic diseases known to be sensitive to high vit C levels, the only side effect of extremely high doses is diarrhea, but the levels officially quoted as sufficient to cause diarrhea, I've never seen to cause diarrhea. Not even twice or quadruple those levels caused diarrhea. Also, in some cases of renal failure or other renal issues, vit C is beneficial. I.e. enterotoxaemia.
This is about the most finite example of a 'upper limit' I have seen, but it's a bit dubious (Wikipedia) and the form of the vit C would be very important to know there:
Plain Ascorbic acid is generally best, but many vit C's contain various additives like lactose which causes separate side effects. Most vitamins/minerals/nutrients naturally come combined with the other nutrients required for the body to process them, so isolated nutrients can sometimes be harmful or just fail to do their job in the body, or cause an imbalance in the other nutrients required for processing which were already present in the body but not combined with the isolated nutrient. Vit C generally does not seem to have that issue, but I am picky about brands and go for clinically tested ones when able.
Vit C is one of those things which is almost impossible to overdose on --- you can go many times over the recommended daily dose without any harm noted. But the daily dose recommended by the FDA many would take exception to, it's almost perilously low. (Playing it very, very 'safe'). Well, I guess under-nutrition is 'safer' than over-nutrition simply because the former kills you slower.

But really they're both just opposite ends of the spectrum of malnutrition.
As an example for overdose levels, I have a sheep who, as a one-week-old lamb, came down with 9 or so adult female paralysis ticks. (If you're not familiar with them, well, it's only the females who harm you, and only as adults; they cause paralysis within anywhere from a few hours to a week or so and by the time you notice symptoms it's often too late. One tick is sufficient to kill an adult animal).
She belonged to someone else at the time, but I was renting there as well; she was orally mega-dosed with about 6000 mg of vit C in one go, multiple times within 48 hours, plus topical application on the bite areas via lemon juice. She survived and recovered movement unusually rapidly whereas a 6 month old lamb from the same flock died within 48 hours from one paralysis tick with no vit C treatment (it was a case of 'trying new things' with the second paralysis tick case).
The surviving lamb was infected with all manner of bacteria from an unhygienic kitchen and feeding practices and by the time I got her she had severe pulpy kidney and a grossly sickened GI tract. I continued to dose her with very high vit C levels regularly and she made a full recovery. As an adult, given a chance, she will scoff many thousands of mgs of vit C in one go, her record stands at about 30 000 mgs in a sitting (before I put a stop to that meal, lol), all with no ill effects. It's been my stand-by for avoiding toxicity when she's gotten into the ornamental gardens or eaten various things I know for sure are very deadly. And she's done that dozens of times now. (She can be quite a Houdini. Plus many visitors have some automatic reaction to sheep or goats, they immediately go fetch any plant they see and feed it to them, without the faintest clue of whether it is safe or not).
As for bentonite, same as with AC you can experience symptoms caused by its extreme absorbency, so constipation etc; otherwise it's much of a muchness with AC, really. As with AC or just plain charcoal, they will often help themselves to it if given a chance. I would sprinkle small amounts around for them, not enough for any one individual to overdose and get clogged up and dehydrated though. Enough for them to taste test and learn how much causes what internal reaction. They will often be able to self medicate safely and reliably if introduced in slow and small amounts so they have time to learn about it prior to letting them have free choice large doses available.
If the bag isn't labeled for animal use I wouldn't feed it to them, but if it's pure stuff and not poor grade theoretically it should be ok, not contaminated or with additives.
My sheep would also make detours into the garden to dig up and eat bentonite that was being used for water retention for the plants, and she would also willingly eat charcoal whenever given a chance, and the chooks are the same. In fact tomorrow I'm off to resupply with charcoal as she's foundered due to getting into too much chook grain plus a laundry list of other unavoidable issues all occurring in combination in the last few days, and AC is one way you knock a laminitis attack on the head.
As for adding vit C and AC, I rarely add them both in conjunction as the AC can possibly absorb the vit C to the point of it being useless, but if you want to use both just to be sure, then I would use the vit C first as it's rapidly absorbed into the tissues, and follow up about 5 minutes or so later with the AC, or any time after that. Give it half an hour if it's not desperate, probably, before adding the AC.
Some people in my family have lymphatic issues, their skin can get as sore as if they've been bruised; topical application of lemon juice instantly takes the pain out. That indicates quite a good degree of rapid penetration of vit C in active form.
Best wishes.