As long as there is not aggressive behaviors developing, they will become friends over time. I would provide an area for the older dog to get away from the puppy when it needs to. (like a separate room or a crate) Also don't forget to give the older puppy attention first, and feedings as well. If the older dog gets alot of treats when the puppy comes into the room or is present, he may associate yummy stuff with the puppy present. Keep them supervised just in case something does happen. What i would be concerned about is the older dog going after the puppy and traumatizing it, which could be a bad thing at this early age. It could have lasting effects.
There are a few factors here. My biggest challenge was potty training as i lived on a second floor. It can be difficult to train because when they have to go they have to go now! Not to mention you have to be aware of the noise because of your close neighbors... whining or barking late at night was enough to pull your own hair out! http://hcgslender.net/ This doesn't mean its impossible, just means there is a little extra work to be done. You have to exercise him on your own outside of the apartments because of no backyard. And i would start crate training immediately as this helps with barking or whining because the crate becomes there safe place.
Actually speaking dogs are very clean animals who avoid spoiling their own area. But a puppy needs a lot of training which may take time and require patience from your side. There are some puppies which may get trained at three months only but others may take a much longer time. Start taking the pup out and wait until he finishes his work. This would be a fun for the puppy and would solve your purpose of training as well.