Question: I was reading how a very small amount of people have perfect pitch and how it's very hard for adults to develop it.

So I was wondering if maybe there's a hack where I can only limit the notes to a scale and then instead of having to choose between 12 notes I could just limit the choices to the 7 notes of a scale.

Is it fairly easy to achieve that with enough practice? also, is it still called perfect pitch to do that or is it called relative pitch?

Answer: 
First, I should pass along that people who have more experience with perfect pitch than I do emphatically state that true perfect pitch (hearing a note and instantly knowing what pitch it is) cannot be learned as an adult. There may be agreement or disagreement, but that is what I'm told, and being in the field of music for years, I've never met anyone who's learned it as an adult.

Second, perfect pitch is not really necessary, and the only real benefit I can see between having absolute pitch and having 100% relative pitch is that perfect pitch makes a great party trick. It is just as useful to learn relative pitch (knowing all intervals and knowing all scale degrees by ear), and that *can* be learned. It sounds like the 7-note scale "hack" you're recommending is similar to learning scale degrees, which is extremely useful and doable.

Lastly, to qualify my statement, what you could learn is pitch *memory*. This is best learned by learning an intro/beginning of a song for each key and drilling yourself on it until you can hear it in that key in your head. Then when asked to sing a C, for instance, you play the beginning of your "C" song in your head, and you can hear and reproduce it. This is not true perfect pitch, and in my experience requires more work than it is useful. Also if you stop drilling the songs, your pitch memory will start to go -- it's not a one-and-done thing. 

Bottom line, train yourself on relative pitch. I have completely accurate relative pitch, and I've never run across something that I absolutely needed perfect pitch to do. 

Hope this helps!
Yes, it's conceivable that this could be a good training strategy. Start by trying to learn a limited range of pitches. Makes sense.

Of course, once you've got ONE pitch right, you're going to find it hard to seperate Perfect Pitch from Relative Pitch.

I suggest you don't worry too much about developing Perfect Pitch. Work on your Relative Pitch - recognising intervals from a known starting point rather than plucking notes out of thin air. It's 99% as musically useful.  

 A degree of Perfect Pitch will probably emerge.