1 Can I mix the coolant
Do not mix coolants. They can have potentially harmful chemical reactive results.
2 Ca I open the Cap when radiator goes very hot
Do not place any body part over or near the radiator when it is hot. Do not open the radiator cap when the radiator is hot. It would be very harmful and may seriously burn you.
3 Does electric system of the car harm the radiator
Yes, Test for stray currents in the engine with a volt meter, to eliminate any chance of electrolysis which can corrode engine parts including the new radiator.
4 How I can get best result after install new radiator
Flush cooling system with flushing agent, so as to make sure there will be no contamination with the new anti-freeze. Contamination can corrode your new radiator as well, so this is important.
After installation please check all plugs are tight and sealed, and also a visual check to make sure all is working correctly.
Use appropriate anti-freeze and correct levels of antifreeze/water mix or premixed anti-freeze. Please note DO NOT mix two types of anti-freeze together as there will be a high chance of contamination which will erode your radiator.