Introduction of one Adult Stray to 2 Adult Males

Hey There,
I just took in a stray who seemed to adopt me (showed up one day and made himself quite at home). He's quite friendly to me and my partner, and I have him isolated for now until his vet visit on Saturday.
He's an adult male, not sure how old (not too old though I don't think). I already have 2 adult males, one of which has always been the "alpha" cat of the household. He's hissed and thrown fits at the window when this other cat has come for visits, so I'm not sure how they are going to get along. Also: my tame cat (nigel) got within 2 feet of this new cat, and the new cat (now named Simon) just lunged and took a few swipes at Nigel. So although he's friendly with me, he may not be with my 2 cats.

Any suggestions on how to acclimate everyone to each other? My other beasties are also only indoor cats, with no front claws. This new cat is LARGE, clearly been outdoors a while and has claws.

Thanks for any help!

Keep them isolated till the new male is neutered (and it takes a month to get all the hormones out of his system). As far as clawed/declawed - that won't matter - you do NOT have to declaw the new one. They can all live together fine.

Then introduce them slowly. Neutered males will get along quicker then most combos but you have an unneutered adult male to deal with and it might take a little longer then normal.

I would try some of these tricks when its time to really introduce face to face.

1. Wash all the cats in the same shampoo so they smell the same.

2. Sprinkle some cornstarch baby powder on all and rub in (to smell the same).

3. Put a dab of vanilla extract on their noses and butts.

4. Get the Feliaway to help neutralize things. I've never used this, but many in here have with good results

Thanks for your reply, there are some tricks there that might work for sure.

I'm thinking he might actually BE neutered already, he hasn't sprayed at all, either outside the house or in. He used his box last night like a perfect gentleman. However, he seems very agressive to the other cats, so who knows, I won't know for sure until I take him to the vet.

Thanks again!

Its pretty obvious on an adult male cat if he's neutered or not. Full grown males have balls - if he's more shorthair you can tell right away

Well,
the new kitty (now named "Simon") has checked out with the vet. He's clean! We've been letting him out with supervised visits with the other two cats. So far, it's mostly a lot of growling, some hissing, and staring. Simon has swatted Nigel (oldest cat) a couple times, and Duncan has snuck up on Simon a couple times and got a swat. But no one is drawing blood, or openly fighting. They sit 2 feet apart from each other looking fairly relaxed.. just... watching.

Simon is VERY talkative though, he's used to being outdoors and yowls to get back outside sometimes. I distract him with toys and that works for a while, or give him a brush (which he loves). Any other tricks to get him acclimated to being an indoor cat?

Looks like I have another kitty!

Also:
are some cats predisposed to being sensitive to heat? It's very hot upstairs (we have air conditioning but it's still so hot), and sometime he pants upstairs. Never seen a cat pant like that...
but he does have a very thick coat.

Just curious..hopefully nothing to worry about!