Start to socialize immediately. But this should be done under ur close supervision. Do not allow them to meet in ur absence for the time being. Labs, specially young ones, are very excitable in nature, so ur pups excitement is natural. Also they r not shy, so he will try to investigate the new crate & the new pet. Let him do so, but again keep a close watch.
Take both the pet in a room. Sit down with ur lab & some doggie treats. Keep the pug in crate & let ur lab meet him through the crate. If he welcomes the pug by licking, wagging his tail, etc then give him a treat. He will be excited for the first few minutes, let him jump & run. Gradually he will settle down. U can also call him & encourage him to sit beside u. Use the treat to do so. When he has calmed down, ask someone else to open the crate to let the pug out. Let the pug explore while to keep sitted with ur lab. U can also use a leash to control the lab, but do not pull him to hard. Let the pug approach the lab. U must not show any excitement/tension, be calm & indifferent when both r meeting each other. The lab may initially try to run away, lure him to be there by throwing some treats. Praise him a lot if he sniff/lick the pug. End the session before ur lab becomes totally excited again. Repeat the session again after a long break. Gradually they will become best friends.
And plz do not neglect ur lab bcoz of the new pet. Love both of them. Sit down with both ur pets & play once they r ok with each other. Keep training ur lab.
Best of luck. Hope this will help u.