The one thing to NOT do is space the other heads further away. The issue with heads on slopes is that a head that is shooting uphill will not throw water as far as one shooting down hill. So if anything, you will want the other heads spaced closer to the down hill rotor to compensate for its limited range.
The reasons for angling a head to compensate for slope is so that the water spray into the hill side doesn't start erroding the hill. But with the hill being such a short distance from the head, the bulk of the water will be shoot past the point of the top of the hill.
So my first thoughts are:
1) Ignore the difference
2) Compensate for the difference by moving other head closer to the corner (by maybe 6"), or
3) use the 12" pop-ups so that half of what should already be a minor difference is compensated for.
Now since I'm making suggestions only based on educated guesses. You could just do some practical tests to see how much of a difference it's really making. Rig up a sample head so that you can power it from a garden hose (may big box stores will have hose to pipe thread converters). Turn the hose on and hold the head near the ground at the bottom of the slope and note how far the water is being thrown. Now go to the uphill side at the far point the head was tossing water. How much of an over shoot does the head have going shooting downhill. That will show you how much to change the spacing.