When you introduce your puppy to the cat, don't leave them alone. Supervision is necessary until they get to know each other. If puppy is smaller than the cat, he could get trounced when your back is turned and vice versa.
During the first meeting, let your cat take the lead. Since cat was in the house first, he gets to examine puppy. Be on the lookout for any aggression on the part of either animal. If the meeting goes sour, take puppy away and try again at another time.
Felines are territorial animals by nature. They may be nice enough to your puppy, but there are going to be places in the house where your cat wants to be left alone. Not respecting the cat's privacy could cause tension between your puppy and your cat. The cat may even get miffed at you and give you the cold shoulder for a while.
Teach your puppy that certain places in the house are off limits. Think of and use a command that you can use to keep your puppy away from those places. You will use it quite often at first but the puppy will learn the cat's "unwritten rules."
Your puppy may start out with something as simple as chasing the cat around the house. This is a normal behavior for a puppy and may even make you laugh. But when the chuckle is over, put an end to the chase and encourage your puppy not to chase your cat.
On the other hand, the cat may catch your puppy minding his own business someday and get him back for all of his crazy antics. If the cat is chasing your puppy, don't scold your cat either. Territorial disputes are normal but since they will both be living under one roof from now on, this can't continue either.
Cats and dogs that live together should eventually come to an understanding. As long as their private areas are respected by each other there should be no problems or any consequences. The puppy's and the cat's relationship may not turn into love right away. But don't get frustrated with your pets because this too will take time.
Your new pug puppy will need to be introduced to your other pets living within the house. If one of the pets is a cat, make the introduction slow. They may not warm to each other at first, but keep it up and eventually they will learn to live together in peace and harmony.