That is an intriguing application. Cement mixer huh? I have experience driving hydraulic motor driven brush cutters, etc. with a much larger tractor so I will offer a few comments that may help you:
1) Look for the FLOW requirements of the gadget. If it says 8 gpm double it. 2) There are many insidious losses -- some you can avoid and some you can't. Size of the hoses (inside diam.) matters A LOT. Other constrictions like connectors and valves matter too. 3) Hyd motor driven devices need to be accommodated with their own idiosyncrasies. For example your cement mixer rotating tub is NOT going to stop on a dime when you throw the lever turning off the flow (!) and that can tear things up if you do not allow for it and accommodate it. You will need some form of bypass or freewheeling capability to allow the tumbler to tumble for a while after you shut off flow. 4) Almost all tractors (and no doubt your 2620) use open center systems of hydraulics. I call that "one track mind hydraulics" because anytime you activate some device in the chain you are also shutting off all the others. For example you will not be able to lift the thing with your 3pt hitch while driving the tumbler. Sudden stops and starts can be either planned for and coped with or they can be destructive. 5) Lookout for direction of rotation. That may or may not be an issue with a mixer.
Wait a minute -- Why not drive the thing with your PTO !!?? The hyd route is going to be far more expensive plus it drags in many grungy problems and issues.
Good luck.