Hi Gabriel,
I have a new puppy who has come into my home with a 12 yr old Tibbie who had a companion until the last 6 months and I was confident he would be happy with a puppy. So he could remain top dog.
The first week was the hardest, but I agree you have to let the two dogs sort it out together in the own doggie language. The less I interfered (which was not at all after the first day or so) the quicker they learn to live together.
I gave the older one the same attention as in the past, but our new puppy Robby obviously demands extra work and play. If it became obvious Daniel had enough playing only then do I give Robby toys and play with him. After the first week they got on well together. IMHO don't change the way you interact with the older dog, and let the two of them sort out their own personalities and level of play together.
RE: food and water. I feed them both 3 times a day and (reduced portion so same amount each day for the older dog). I take the food outside for Robby (puppy) so that he can wee and poo afterwards. Daniel gets fed inside out of site of Robby, so he can have his peace with his food and then go out when finished. As far as water I have numerous bowls around the house and let them sort out their own favourite place to drink from. Although my two don't have a problem sharing water cushions etc. Sometimes I think they check out the water after the other has drunk because of smell or something. Like just checking up. If your older dog is not dehydrated maybe don't try to make him drink from you. If you are worried about dehydration, syringe (no needle tip) water or electrolyte into their mouths (like giving medicine to a child). They get the message and start to drink more themselves.
Hope this helps
