Adding noise to an ECG signal
1) Create a 50 Hz sinusoid and then simply add it to your ECG signal.  You can control the power of the 50 Hz noise by multiplying the sinusoid by some gain factor (can be less than or more than 1) before you add it to the ECG.

2) I'm not familiar with the Welch periodogram, but if it displays the power spectral density then it should do fine.  I would just do an FFT myself.