Congradulations on your new baby. Connie is 100% right. He is tiny so you need to watch for a low blood sugar. As long as he is feed good food regularly and gets lots of sleep then he should be fine. Good food doesn't mean steak and potatoes, good food mean a good dog food
Another common mistake is people get this cute new puppy and want to play with it all day long and forget that it is a new "baby" and babies sleep
alot just like a new human baby. I learned about hypoglycemia (low blood sugar) the hard way. I was very lucky that he didn't die. No one ever told me of any such thing and my little girl had never had such a thing. I think that it had a lot to due with the stress of all the change too. 3 or 4 days after I got him I woke up to what looked like a dead puppy in his crate. He I known what to watch for it would have never happen. It never happened again, with any of my dogs and if it would have, I would have known what to do. As for Kirby, he needs to be watched closely with Gizmo. It sounds like he might be trying to play, but he can hurt him badly without meaning to. They need to interact together so they can be used to each other, but only under supervision until Gizmo is a little older and isn't as fragile.