Hey!
Welcome to AN

Normally I'd advise against doing a 7th subject, however in your case it's a bit different. I don't think it'd necessarily be a bad idea depending on a few things:
-Is your school going to give you a hard time for doing the bare minimum in religion?
-What is the bare minimum? Will you only be doing work in class? If not, how much time out of class will you have to spend on it? Are you going to get heaps of compulsory homework?
-Is there another subject you want to do?
Obviously none of us can decide for you, but I think it'd be worth considering the above points. Providing that you can get away with not putting much time in it, and there's another subject you want to do, then I'd say go for it

(btw, religion is compulsory in my school, will it be put in top2 of my top4 subjects just like how English does for atar calculation?)
Nope.