I see nothing wrong with having different breeds of different sizes, or different species together. Since both dogs are now puppies, they will think of each other that way...siblings...growing up. The actual personalities of the two dogs will determine how well they get along, as well as how you handle them growing up. Whether or not they are altered/intact will also play a role.
I have raised different sizes together, and felines with canines, with no separation and no issues. In fact, the cats would become an 'irritation' to our late 120+ lb. black Shep/Lab mix, Ziggy (Zigzag), by snuggling on his paws and licking his face to the point where he would finally move away just for peace.
Little doggies can sometimes act like they are at least as huge as whatever other animal they are 'expressing their opinion' to...they have no idea they are so small. My 6 lb. future-grand-pup 8-yr-old Chi 'herded' my Shep/Lab around the room yipping at him and snapping at him...and poor Ziggy'd just go find someplace else to hide from her...even leaving his bed near us and going to another bed in a different room.
She only stayed with us for about 6 months, but we never separated her or the cats from Ziggy, although they would separate themselves to comfy spots, and never the first issue...ever.
If there is ever any doubt at all, or if you just want to make sure the little ones are not accidentally harmed when you are not around to supervise, just separate them. Better to err on the side of caution.
It is entirely possible, however, that you will not have any serious issues, unless things go hugely wrong...
