I would not mix alcohol with Nyquil. That can be dangerous. Can damage liver I think and I heard that when you "combine alcohol and ANY downer (Nyquil, in this case), you run the risk of seizures, coma, and death."
Also, you shouldn't mix NyQuil with Tylenol. Tylenol is a brand name for acetaminophen. You should look into information about a drug before taking it, prescription or not. Under the label of NyQuil cold/flu it mentions that you should not mix NyQuil with acetaminophen, because NyQuil already has quite a bit of it in it.
Here's a nice site with info about Nyquil Liquicap (allergic reaction symptoms as well as a list of drugs that can have an effect on Nyquil Liquicap)
some notes from this site....
"Avoid taking diet pills, caffeine pills, or other stimulants (such as ADHD medications) without your doctor's advice. Taking a stimulant together with cough or cold medicine can increase your risk of unpleasant side effects."
"If you drink more than three alcoholic beverages per day, do not take acetaminophen without your doctor's advice, and never take more than 2 grams (2000 mg) per day."
"Do not use this medication if you are allergic to acetaminophen, dextromethorphan, doxylamine, or pseudoephedrine, or to similar medications such as other decongestants, diet pills, stimulants, or ADHD medications. Do not use this medication if you have used an MAO inhibitor such as isocarboxazid (Marplan), phenelzine (Nardil), rasagiline (Azilect), selegiline (Eldepryl, Emsam), or tranylcypromine (Parnate) within the past 14 days. Serious, life-threatening side effects can occur if you take a cough or cold medicine before the MAO inhibitor has cleared from your body.
Before taking acetaminophen, dextromethorphan, doxylamine, and pseudoephedrine, tell your doctor if you are allergic to any drugs, or if you have:
alcoholism or cirrhosis of the liver;
heart disease or high blood pressure;
a thyroid disorder.
If you have any of these conditions, you may need a dose adjustment or special tests to safely take this medication.
FDA pregnancy category C. This medication may be harmful to an unborn baby. Tell your doctor if you are pregnant or plan to become pregnant during treatment. This medication may pass into breast milk and could harm a nursing baby. Do not use this medication without telling your doctor if you are breast-feeding a baby.
Artifically-sweetened liquid forms of cough-and-cold medications may contain phenylalanine. This would be important to know if you have phenylketonuria (PKU). Check the ingredients and warnings on the medication label if you are concerned about phenylalanine."