Person in vocabulary

[Leebo]

人 Is there any rule to readings? Also a question about same readings

There are rules for when 人 is a suffix. Nationalities じん, counting people にん, person who does an action にん. This does not include times where it just happens to be the second kanji in a normal kanji compounds. It’s just those situations as a suffix where there are rules. The other times between じん and にん are determined by when the word was imported from Chinese into Japanese. So you need to come up with a different way to remember them, or just rely on exposure to solidify it. As for homonyms…

To add to what I said there, some people create characters who are always used in associated mnemonics, like a ninja for にん and a genie for じん or something.