You can mix warm noodles with mayo if you are serving or cooling them soon after you mix them. Because warm mayonnaise can provide a breeding ground for potentially hazardous pathogens, it's better to use chilled noodles if you can. Warm noodles also have the potential to make your mayo runny. If it collects on the bottom of the bowl rather than sticking to your noodles, your salad won't be as tasty.
Cold Noodles, Warm Noodles
Most recipes that include both noodles and mayonnaise are salad recipes, which should be served cold. Although cooling the noodles seems to create an extra step, you'll have to cool the noodles anyway after you add the mayo if you're aiming for a cold salad. You can cool noodles in the refrigerator, but this may make them gummy unless you mix them with plenty of oil before putting them to cool.
Cooling Noodles Quickly
Instead of oiling noodles and cooling them in the refrigerator, you can simply run cold water through them when they're draining in the colander, continuing this process until they cool to room temperature. This cooling method only takes about a minute for a pound of noodles. Running water won't cool noodles to 40 degrees Fahrenheit or colder, which is the safe cold holding temperature, but it does give them a head start so it only takes a short refrigeration period for them to fully cool. Cooling noodles with water also allows you to use less oil during the final part of the cooling process because the rinsing takes much of the sticky starch off the pasta. Shake the colander well before transferring the noodles.
Mayo and Food Safety
Although mayonnaise is widely thought of as an ingredient that can turn toxic or deadly if kept at room temperature for too long, commercially produced mayonnaise has enough acid to keep it from being a serious food safety threat. In addition, the eggs in commercial mayonnaise are pasteurized and unlikely to harbor dangerous pathogens such as Escherichia coli -- lovingly nicknamed E. coli -- or Salmonella. Because noodle salads with mayonnaise may contain other ingredients, such as vegetables, you still need to make sure you're treating the salad correctly. To be safe, chill the noodles before mixing in mayo to avoid creating an environment where pathogens can thrive.
Other Noodle Salad Dressings
While mayo can get soupy when mixed with warm noodles, other dressings may fare better because their ingredients start out as liquids. Using a liquid dressing allows you to judge when the noodles are coated. In contrast, mayonnaise-based dressings may stick to warm noodles when you first add them, but run off as they warm. Try dressing noodles with olive oil and lemon juice, or soy sauce and toasted sesame oil.