There is no rule, they mean the same thing

Maaf, Kamu salah.

When should I use kalian or kamu? Is there any rule, or can I use either?

'You' could be translated as many words in indonesian based on the contexts. 1. It is translated as 'anda' or 'kamu', IF there's only 1 person you're talking to. 'Anda' is more formal. 2. It is translated as 'kalian' IF there are more than 1 person you're talking to.

So, 'kalian' is a plural version of 'anda' or 'kamu'. Meanwhile in english you can use 'you' both as singular and plural.

silap?