I don't think there is one clear answer for this, there are too many different variables at play.
Ammonia becomes increasingly toxic as the temperature rises, and also as the pH increases. Because of that, what is toxic and what is still tolerable can vary from tank to tank. The amount of natural protection an animal has will also play a part (Scales, gill covers, overall health of the animal, etc.) Generally speaking, I think cold-water species like goldfish will be tougher than tropical species (not just temp, but pH is higher in many tropical waters too.) You also find more scaleless fish like puffers in tropical systems- doubly bad for them, who lack any real bodily protection. Axolotls are a bit harder to place, because they do live in cold, fairly neutral pH waters, but their bodies are quite fragile. To give an exact number on what is toxic... I really couldn't say. Wouldn't the period of exposure also have an affect? I mean, isn't lengthy exposure to low levels of toxins as potentially harmful as a short exposure to very high levels? To me anything above zero is a red-flag- you could try emailing the Indiana University Axolotl Colony and see what they say. They've helped me out with questions before, and maybe somewhere in all their research they've done some studies on ammonia/nitrite tolerance limits?