Since all cats are adults, I'd leave the new ones be for at least 24 hours for them to get acclamated. Leave a radio or tv on for them. Rub them down with an old rag and let YOUR cats smell their scent. Do the same thing with your cats. This way they will all know that there are new kids on the block. I assume they are all neutered. If not, I strongly suggest it. Unneutered males can be very aggressive.

Go in and spend time with both of them. Lay on the floor and give them treats, gently talking to them. They will probably hide, that's normal. But they'll get used to you and their surroundings the more time you spend with them.

Give them a couple of weeks. Then, if you have baby gates, or know someone who does, borrow them and stack 2 of them one on top of the other in the doorway. This way they can see and smell each other and get acquainted safely. Once you feel comfortable in that they can be let out WITH supervision, take the gates down and see what happens. Also, have a squirt bottle handy, should anything erupt. There will be hissing and spitting, that's normal. Don't squirt them unless a serious fight erupts, then separate them again. Your cats will show them THEIR order in the crew.

Patience is the key to gradual introduction of 2 new adult cats into a home with other resident cats. It'll probably take a while, but then again, they may surprize you and be the best of buds. Cats are strange creatures of habit. They don't like change.

You'll also need one cat box for EACH cat, as well as food and water dishes.

I wish you luck. That's great you've offered them furrever homes. Hopefully, other members on PT can offer you advice that I left out.

Welcome to Pet Talk!!!