What do you mean by that? Are you saying you want an inverter that is sized to your solar/hydro inputs? Wait, I just went back and skimmed some earlier posts, are you talking about grid-tie here? I cant really weigh in on grid tied, as I have never gotten into that, but in truly off-grid applications the size of your inverter is going to be dictated by the loads you want to run, and maybe also by how much amperage your batteries can sustain. Did you end up getting those 800AH 24V forklift batteries?
Garry is right, solar panels do not generally need diodes, and I am pretty sure charge controllers will prevent back-feeding anyway.
I have used blocking diodes before, but it is really only an issue if you are hooking a DC generator directly to the batteries. I will say that this will probably only be the case if your hydro is really close to your batteries, or you build some sort of DC generator. If you have to transmit the power any distance, 24v is going to call for big-ass wires, and then you will probably want to up the voltage and probably use some sort of hydro charge controller, which will have its own blocking diode in there. When I built my pedal powered generator I plugged it directly into my DC system, and it would have turned the generator without the little 6A diode I put in there.