For yokes i would consider using 120mm sq box section with a flat plate welded on the bottom to take a 450mm concrete panel below yoke or if shuttering a wall you could use 152x152 u/c there are a few types of end brackets you will need the correct one for your choice of post,i have set my posts in 700mm deep and no problems .The yokes will need a centre bracket to support them mid span, what yokes are you planning on using ?Replacing old feed fence with some 6m locking yokes for dairy cows.
Two questions;
1. What size of RSJ is required
2. How far in the ground does the RSJ need to be?
Thanks
Alex