Introducing rats to each other can be stressful sometimes, and other times it is quite effortless. I have been through both - where rats ended up at the vet with bites taking weeks to integrate, to tossing rats into an established cage and they hit it off right away and became life long companions.
From what you have written, it is likely that the aggression and dislike for other rats may have stemmed from being in such a small space and being treated poorly. Rats that do not have enough cage space, food and are in poor health tend to be far more territorial and aggressive. The best way to introduce new rats to each other is by doing so in an open space at first, like on your bed or couch during playtime - a neutral space that is not in their cage (their own territory). You'll have to see how introducing Yeti to another rat goes, gradually.
It seems that if Yeti was living in bad conditions that a part of his dislike for other rats stems from his past - if he is in a happy place with lots of space, good food and clean water with comfy clean nests and cage with things to keep him happy he may very well change and improve in temperament.
Rats do tend to be social animals, and while it is hard to integrate rats together at times, it is really for the best as they really need rat-to-rat contact and friends to thrive.
I have often done the following:
- Put the new rats and old rats in separate cages that are next to each other - close, but not close enough to allow contact through the bars. This way, they can become aware of each other and learn each other's scent.
- After about 2-3 days, I'll take them out and let them meet each other in a neutral area away from their cages. If things go well and their are no fights (rats will sniff, hump and sometimes wrestle each other, if there is loud squeaking it is a bad sign as it means they are hurting each other - separate them right away).
- Let them have lots of playtime in a neutral area so they get used to each other, after about 5 days of regular together time, it is time to integrate them.
- Finally, if things are going well and they seem to be happy with each other, put them together in one of the cages after giving it a thorough wash and clean to remove as much territorial scents as possible. Provide at least two nesting/sleeping areas so that they can sleep together and apart if they want. If things go well, then you'll have integrated your rats happily.
Sometimes, older males who have been alone for a long time struggle to integrate with other rats, especially male rats, due to their own habits and nature. To some rats, being integrated with other rats can be unsuitable, but you won't know unless you try it. You didn't mention if you have an idea of how old he is, so I am not sure how old he is.