Will changing governors causes any stability issue?
Yes it can cause, it depends on your hardware, kernel and your style of using the phone (AFAIK). Also it is good to think what job scheduler you use with your cpu governor.
For me, lulzacti...
I think not possible, because P5210 is Intel Atom (x86) basis and Tab 4 is Qualcomm Snapdragon (ARMv7) basis. So firmwares of these are not compatible with each other. Maybe there could be "Tab 4-alike" custom firmware for P5210, but it needs some...
Is there a battery out there around 3000-4000MAH that doesn't make your phone any bigger?
I say the same as some on before, they're probably fake ones. Currently battery technology cannot go smaller, so if you want more mAh, then physic...