Why do we perform Gunna only on Meem Mushada & Noon Mushada, is there any specific rule?
The ghunnah is an intrinsic quality or characteristic of the noon and meem. The ghunnah is a sound emitted from the nose, nasalaization.
You cannot say the noon and meem properly without the ghunnah; this is exemplified by someone who has a cold. When they pronounce the noon or meem, the sound is imprisioned and does not sound normal.
The extended timing of the ghunnah exists for the noon and meem with a shaddah, but also with the rules of ikhfa' and iqlaab, but it is longer with the shaddah.
The following links should give more information about the ghunnah: