Stray cats can get on even when confined to a relatively small space. I should know as I have three stray cats in an apartment! Well Binnie is no longer a stray cat, she is a long term companion of mine. But Timmy the alpha boy cat and Pippa the girl all get on now. It really is a question of time. I see people asking questions about introducing a new cat to a household where there are already cats. There can be problems and certainly there can be initial problems but with patience and a little human intervention and management things gradually settle down.
At first Timmy would hiss and strike out at Binnie and indeed Pippa but right now he has come in from the cold and plonked himself on my bed as I type this and he is about 12 inches from Binnie. He just sniffed her and Binnie made a little trill and that was it. He just curled up and she got comfortable.
I think Timmy has come in because he is still a little poorly after the visit to the vet to lance, drain and treat his bite wound. See Cat Fights of Male Cats and Cat Abscess. Stray cats can get on, just give it time with a bit of temporary management.