My Labrador is about a year and a half old now and we have had him since he was a puppy. We recently brought a pug puppy who is very small compared to our Labrador. Our puppy often tries to bite our Labrador but when the Labrador play fights back, as he's so much bigger it looks like he might hurt the puppy. The Labrador always wants to play and keeps chasing the puppy and as he is so much bigger he ends up knocking him to the ground and trampling on him. We have to always watch them to make sure it doesn't get out of hand. Will this stop as they get used to each other and won't get so exited around each other? Can i do anything to help or will they always be like this, because that would mean I may have to sell the puppy so one doesn't end up getting hurt, plus it's frustrating having to watch them all the time.
They're being more careful than you give them credit for. If one of them gets a bit too rambunctious the other will tell them so and it'll correct itself. As long as it's play, your only concerns are keeping them from knocking things over, telling them in no uncertain terms when this is not the time for being silly, and keeping humans who might misunderstand from wading in and confusing a situation that the dogs have well under control.
I agree with Keshlam but wanted to provide an answer for anyone looking to train their dog to play softer.
I would recommend watching some videos about dogs playing, especially if they are narrated, so you can watch the dog's expressions and hear what they mean, and listen to different scenarios.
Watch the dogs carefully. Don't interfere, just watch. When you feel that they are being too rough, call them both to you, give them both a pat (to reward them for coming to you), and then let them both go play again. That's all you do, no words or scolding needed. Repeat the process a few times; this also trains your dog to "check in", which is great for going to the dog park.