DROBO connected to a mac mini over the network, how do I manage my storage within OS X (free apps)?

I’ve considered connecting via USB and using a virtual machine to house a server OS to manage permissions on the storage. However, I’d take a further performance hit. I like to back up my macbook pro over the network, but if the DROBO is set as a single partition, there isn’t a way to limit space allocated for backups. Then I tried setting partitions on the DROBO, 2x what the size of the computer, so 500GB for my MBP, and 640 for my mac mini. Time machine is set to record over oldest backup, but instead gives error that no space is available. Apple “Geniuses” tell me I need a copy of OS X Server. Any ideas?