Why is it recommended to be able to do 10 pullups before adding weight? I never really understood that.
Most people treat pullups like some kind of special exercise where reps, sets, and weights are somehow on a different plane of existence and doesn't obey all the rules of fitness.
It's no different than any other exercise. You will increase your strength no matter what your rep range is, but you will increase your strength more if you do weighted ones in the 3-6 range, get more hypertrophic results in the 6-10 range, and get more muscular endurance in the 10-18ish range.
So, what are your goals?
Personally, I do think it's important to have the muscular endurance to do pullups (or chinups) in the 10-ish rep range. But honestly if you can do 5 weighted pullups with a 25lb plate chained to you, doing 10 with just your bodyweight will be cake.
Also, regarding one of your comments:
It usually takes me 30 minutes of rest between sets of 7.
That's a lot of rest, like basically multiple workouts. Not that that's a bad thing.
It's not an official strength standard but I think there's a bit of wisdom to the idea that 3x10 (or 3x12) chinups/pullups is a good idea. Pullups/chinups are multi-joint exercises affecting the back, trunk, shoulders, and arms. Having muscular endurance in that department is akin to be able to run a mile: it's just a handy thing for a human being to be able to do. For a variety of reasons.
All that being said, introducing weighted pulls/chins "early" isn't going to do anything but help. Just don't ignore the value and significance of the exercise for developing muscular endurance as well. In a year or two if you can't do 3x12 with a minute in between, you're probably missing something.