Try rock chunks of various sizes (They came with the game, found under destructibles). Their "grey" color tends to blend well with most landscapes.
Also you do not have to have your rocks sitting on top of the land - lower a few of them into the ground.
This is more of a plain than your "Plains of Peace" motif.
When it comes to rocks in the plains - it is best to keep the rocks on steepish hills. Most plains have been growing grass for a long time - ending up with several feet of sod over the soil. Sod is a mat of roots usually of grass intermixed with partially or wholly decomposed organic matter (the grass that lived on before).
When one has hills in the landscape that are steep one should assume some erosion has taken place, thus the ground has shifted and half buried low lying spots - mix it with grass you should have low rocks, or mostly buried rocks except where erosion has exposed them, like on steep hills.
Hills are created either through upheaval in the land (like mountains) and have eroded down over the eons or are what is left behind from eons of erosion of a flat land. In some cases they are deposits from glaciers.
Hills created by upheaval and through erosion are usually dispersed at random - glacier deposits end up like wave marks in sand upon the shore - lines of hills. Glacier created hills are commonly called Moraines:
http://www.arctic.uoguelph.ca/cpe/environments/land/glaciers/features/moraines.htm
The First would have exposed rock on the hills, the valleys would be infill - what rocks are exposed would be of the same kind. Exceptions would be made for streams and gullies where erosion of fast moving water would take away the lighter smaller debris, leaving behind the larger rock and stone.
Moraines the later could have boulders strewn across the landscape however the rocks would come from many sources thus would be a jumble of types/colors.
I tell you all of this to give you some idea what people "expect" to see in a landscape. As a species we have come to expect certain aspects of geology - where and when geology offers something different or out of the ordinary people see the land as "wrong" in many cases folk can not put their finger on what is "wrong" with the landscape.
I find that a lot of the time when people point out "flaws" in the terrains offered their "gut" is telling them to expect certain natural processes to result in certain effects.
I would suggest that you spend some time looking up images on the net dealing with landscapes. EXAMPLE, if you are looking to create a desert landscape you would search the word and get many hits - something like:
http://images.ask.com/pictures?qsrc=167&o=0&l=dir&q=desert lots of images to draw upon for inspiration. Just a quick over view and you get a "feel" for what a desert looks like.
