If you are planning to cool to pitch temps in the mash tun, I’d say 1100-1200g working cap.
If you are only planning to partially cool in the tun, and add water to the fermenters, you could get by with 1000g working cap.
I’m not sure if what your asking is, should you go below 1000g capacity. You could go slightly smaller, but I think that might be restrictive from a process perspective. Say for example, you were doing a 100% rye mash, you might want the full water volume due to viscosity challenges.
We chase our pumps and pipelines with water when we transfer, mostly because it’s cleaner for us than having to open a hose connection and have gallons of mash spill on the floor. For our 500g batch sizes (full volume in the tun), this is usually about 15g of water for rinse/purge on the transfer.
We’ve played around with mashing heavy mashes, roughly 400 gallons total capacity, 1100-1200 pounds grain – to be able to use large chilled water additions to speed cooling (pre-filled the fermenters with water and set them to cool to around 45-50f), we found the product yields were slightly worse than working closer to full volume through the cook/malt additions.
What did work better for us, is going heavy on the corn gelatinization, then using water additions to bring the mash down to malt-drop temps very quickly.