Introduce new cat to old

I adopted Gigi in April. She is 10 years old, good health and adapting well. My mother is in a nursing home for probably 1 month and I took her house cat home for foster care. I was visiting the cat every day since this happened Monday to feed him. I live 45 minutes away and decided it would be easier on me to take Poncho home with me yesterday afternoon. I live in a small apartment. I have read some things about introducing a new cat to another cat and home. Poncho is about 8 years old, healthy male.

Poncho is staying in my bedroom with water, food, litter pan. Last night he was restless to be sure. He and Gigi have seen each other briefly, hiss, snarl and spit. Both cats took a swipe at me and landed. What do I do? I have to leave them alone during the weekday to work. I will go straight home from work and have this weekend to sort out things and just be there.

Poncho gets snarly and scratching at me when I am about to leave the room. This morning, he was so friendly and cuddly, when I got up to leave, he scratched and really went after my ankles. What do I do to ease his stress?

Gigi was in better spirits this morning already. She let me pet her, was acting normally and sniffing at the bedroom door. I almost think she is trying harder to accept than Poncho at the moment.

Im dealing with the same thing, I leave Mugs"the new cat but older" in the bedroom with his food and litter box, When I get home I let him out and watch them. Mugs is bigger by 2 times, but Otis is a killer and a little crazy. If things get nasty I'll put Otis in the Bedroom and let Mugs explore. I've been sleeping on the sofa so Mugs can stay in the bedroom and Otis doesnt feel ignored with me locked up in there all night. So far only 2 battles, one got bad but I stepped in. Today was the first day Otis let Mugs come near him and sniff him a little. Little by little its getting better. Its working for these guys, and both have always been the ONLY cat in the house till now.

Keep them separated, with food and litterboxes for each. You can help them adjust to each other's scent by rubbing towels over them and letting the other cat sniff the towels. To reduce their stress, you may want to get some Feliway spray or Feliway diffusers. Feliway is a calming synthetic cat pheremone. Other people have had success using Rescue Remedy, which is a Bach Flower Essence. It can be rubbed into their ears (just a drop or two) or put in the water. Again, it helps calm kitties (and people) to reduce their stress.