In my opinion the Random ladder should be implemented without rewards for a short time, to see if it works ok ,and what does it cause in the game.
There are many multiples options for the rewards, but i think a good option could be like the new NPC that gives u prizes for pvp missions, as winning 5 in a week, or reach 100 rating. So the rewards could will not be exaggerated and people will not abuse this way to farms coins or rewards. And for the top 25 ladders, the total of pvp coins should be 20% of the normal OU ladder, cause u dont expend time or money trying to get decents pokes for pvp, or expending in the movesets or rerolls.
As someone mentioned before, getting berrys would be great to, but not for each battle, maybe getting 1 berry after 2 wins or something like that
And about the rotating between OU and UU, i think is a bad idea, not only for the pokes, i think many would stop pvpeing when the UU arrives for the ladder and the queue will be longer, so i think should be implemented separated from the OU ladder.