If we hold a portfolio of stocks shall we need only consider the systematic risk of the securities?!?

Answer 1

That would depend on the portfolio of stocks. With a sufficient number of stocks, it is possible to diversify out the security specific risks of the stocks and leave only systematic (beta) exposure. I beleive 30 stocks is the commonly accepted minimum number, but you would have to choose them pretty carefully to make sure they weren't too highly correlated with each other.