Scr triggering design, shuld i use four pulses for each SCR ot two for each pair of the SCR?
Answer for How to Trigger the SCRs.

You have to trigger U6 and U5 simultaneously.  This causes current flow in the direction of u6-load-u5-neutral (return path) during Positive half cycle of input. 

During Negative half cycle of AC input SCR U6 and U5 turned-off. Due to Natural Commutation.

And then you have to trigger U3 and U7 simultaneously. This time current take path U7-load-u3.

You can control the output Voltage by adjusting the Firing angle of SCRs

And also UJT Relaxation Oscillator with Pulse Transformer is good choice for firing this SCRs