I guess if you fight a bit then some tough people will want to be friends with you.
Fact is, that even if you lose. If you fight back and hurt people a bit, or initiate a fight and hurt them.. They prob won't want to pick on you.
Of course no point looking for quiet "nice" people because that won't offer any protection.
I reckon prison works the same as school. There may be 2 types of bullies. One that picks on the person he thinks is weakest, because he thinks that person won't respond, and so it's an easy way for him to look tough. There may be another type, but most are picking fights where, it's not that they think they will win. It's that they think there wont' be a fight at all. So if you fight them, they won't pick on you.
In school people would go away with just a push, because it made them look ridiculous. The same would apply in prison of course. Though you may want to do a punch instead. Even a verbal response would work in school.
Anything that blocks the bully type from making himself look feared and tough.
There are loads of these types in prison i'm sure.
Just be a bit strong, unattractive, and maybe, maybe, go around in a vest . (depending on whether going around in a vest makes you look tough than others, or more like a queer)