Possibly adding to Pack

I have an 18 wk old male Lab pup, who is a good boy for the most part but from time to time plays to rough with my 3yo female Bichon. Would it be a wise decision to add an additional pup to our pack, so the Lab would have someone more his size to play with?

Not yet.
   
You Lab is under 5 months old, he's only a puppy. He's not trying to hurt your Bichon, he's just enthused. It's too early to decide on another dog.
   
The Lab will calm down as he gets older.
   
Put a bell or something that rattles on the Lab's collar, so the Bichon hears him coming & can get out of the way. Give the Bichon a box or somewhere else to retreat to when necessary.
   
And given time, the 2 of them will likely work things out & get along fine.
   
Just keep an eye on them when the Lab gets rambunctious & help the Bichon get away when she wants to. Take the pup out on long walks to wear him out, so he's more tired in the house.
   
It's like having a toddler wanting to play Tackle...