I noticed that आमिर तुझसे लंबा है is also possible. How is the meaning changed if I leave out the ज़्यादा ?
Meaning will be same in both cases.
When I lived in India, in using comparatives with से, people would just drop the ज़्यादा. For instance: आमिर तुमसे लंबा है। Is this also grammatically correct?
Yes, it's also correct.
और लम्बा ≠ ज़्यादा लंबा?
और doesn't work here, it can be used to mean 'more', as in मेरे पास और पैसा है (I have MORE money). But when comparing two things it's ज़्यादा.
So is "aur" more like comparing quantifiable things as opposed to qualities then?
Why आपको is not accepted?
you need से for comparison not को
it's aapse not aapko