Hi India,
thanks for explaining your reasons not to parcel the sim. I understand the interest of the builders that they need space and prims for their creations. But you should consider that there might come up an issue when your sim is filled up with renters what hopefully will be. Because you have to reserve the promised amount of prims for the renters, this limits the amount your content builders have available. Or in other words it might happen, then that a builder rezzed something very big and a renter has no prims available to rezz in this time.
So my suggestion is the following scenery which covers all the interests.
1. Make parcels around the sim at the water side to rent out. Advanage: The renter is able to change the parcel settings as far as you didn't limit in estate manager window.
2. Give a primbonus to the sim (about 20%). Advantage: So you can have smaller parcels than they would normally be and so the area in the middle becomes bigger for the builders,
3. In the middle area you can provide a community place for renters and guests that they can have party. Of course you can set it to the goup of residents that they have permission to rezz or maybe leave open for guests to rezz, but then with auto return that you dont need to clean up.
About the terraforming: You can disallow this in estate window, but in my opinion its not required. Its possible to bake the terrain so you can revert it in seconds if someone might mess it up.
If you are interested please have a visit on my sim called "Niko". Ok its a question of personal taste if you like the style there. But it shows exactly what i was trying to explain. In the middle there is much space for guests or renters to meet and have party, and in the air above i have much space and prims available to build and i even rent out skyhouses there.
Greetings, Marc.