I don't think the stones are too large at all. I think that quite often the problem with some of these style aquarium is that the stones tend to be too small! Bigger stones make more of an impact to me. The smaller ones seem incidental.
Choosing stone to aquarium size is tough. Personally, I think when you find a stone that you feel is big enough...go bigger!
The bigger stones just look more impressive and fill out the aquascape. I'd rather see two or three big stones than seven or eight small stones.
As far as breaking up the stone, I can't answer how the Ohko will respond.
I will say I wouldn't break up the stone. Instead, I think you should spend a little more time trying to lay them out (as if you haven't agonized over this already). The stone on the left is very vertical. It looks very tall. The stone on the right is squat. The others are lying so flatly they disappear.
Have you tried the grouping of three? The three largest stones laid out in a triangular pattern? That may help a little.
You can pull this off with the stones you have , Kevin. I am sure of that!
I think it will take a little more time, maybe a couple of days of placing the stones and coming back to the hardscape later on, but you can make this perfect with what you have there!