it depends on the situation and the relationship with others . most of the time , ni chi le ma ? use to a person you already konw for each other , not for a stranger . but if you have a very close relationship with a person , you will skip the greeting , say the things you wanna say directly . so i think ni chi le ma ? can use to a person , who you know for each other , but not very colse , such as your neighbor , when you use ni Chi le ma , you don't need to say hello ,before it , it can relpace with ni hao, cause when you meet your neighbor at a supermarket, say nihao, show distance , your neighbor might think , hello , i am who live near your house , not somebody you don't know . so in this situation , you can use ,ni chi le ma ? directly ,to start a conversation .

But after "nǐ hǎo” we don't say " Nǐ chī le ma?" any more. After "nǐ hǎo” you can say " (你) 最近怎么样啊?" or " (你) 最近还好吗?" etc. ( it means " how have you been?". )

No, it's very outdated.

if u say "ni chi le ma", and Chinese people will think, oh, u are a good language learner coz it's warm and local. if u say"ni hao/ni hao ma?",it would be more polite, or u can just give people a smile, the body language can be understood easily.

Molly has answered the question quite thoroughly, so I just want to add a few general points about Chinese greetings. In general, Chinese culture is more situational and context-dependent than English speaking cultures. Just from the examples Molly gives, you can see that there's a range of situations in which 你吃了吗 / 吃饭了没 is appropriate. We Westerners learn "你好“ and assume that it's equivalent to "hello". In some contexts it is, but 你好 does not mark a close relationship, and is said by far fewer people than we might think. After living in Taiwan for a while, I noticed that a common greeting of neighbors and co-workers is to make a statement about what the other person is doing, for instance, 你来了 (you've come!)， 上班咯 (going to work!)， 下班咯 (home from work!)， 出去玩 (you're going out!)， 你吃冰淇淋 (you're eating ice cream!)，and other such observations on what the neighbor/colleague/etc. is doing at the moment.