Cats don't necessarily like each other upon introduction. They're territorial, and in a "multicat" situation, an alpha has to be determined. Even with slow introductions, with two strong personalities, there will be fighting. Also - we as people can mistake play for fighting - though hissing and/or spitting and ears back are not good signs. Batting on the head is a way to establish who's boss. Fluffy tails and ridged backs CAN happen during really active or intense play. Play can turn into a fight, however, it happens here, and these guys have been living together for two years.
But just because introductions can't be made slowly doesn't mean they won't become friends! You just can't ever tell with cats. The slower intros give things more of a chance to work - for you and them. But we don't live in a space that allows for segregation except for in a VERY small bathroom. So new kitties didn't get left out in the general population until we were sure no one would get hurt when we were gone - and we just supervised interaction while here otherwise. We did use Feliway and vanilla - Feliway to help calm the environment (which helped with our situation), and vanilla rubbed behind their ears and on their rumps for the first few days to help make everyone smell the same.
Lazlo is still a loner. Spooky and Flowerbelle and Sheldon play, play-fight, groom each other, and cuddle with each other. Tuxedo only tolerates Flowerbelle. It's just his personality. Only time will tell - but only Flowerbelle had a separated intro. So just do your best to adapt your situation - and if they're fighting and it looks like they're going to hurt each other, rattle coins in a jar at them to break it up. Do NOT physically get involved in breaking up any fighting. Startle them with a noise - clapping hands should work too.
But if you can invest in a baby gate, having a little time to get used to each other without actually being able to fight because they're separated will help things go easier - but not being able to do that doesn't mean they won't become fast friends. It'll just happen on their timeline, not yours.