Yes, as you wrote, it is compatible, just a small adapter needed fw800 to fw400, and this only if the OP plans to use the monitor's fw400 ports.I think the FireWire 800 port on the Mac Pro 5,1 has backward compatibility to the FireWire 400, isn't it?
will I need any special adapters? I have the older 20 inch silver cinema display, will it connect to the mac pro?
You might be able to get it done with 4 of these...
http://www.amazon.com/VisionTek-Radeon-7750-GDDR5-MiniDP/dp/B00C7EPSVS?ie=UTF8&keywords=eyefinity 6&qid=1461603069&ref_=sr_1_1&sr=8-1
UPDATE: Oops, sorry, I misread this as I though you wanted to plug in a quantity of 20 displays! Nevermind.