So i've heard that its not a good idea to throttle your pump output with a valve. That being said the Pondmaster im considering is oversized and does not have adjustable flow, so im wondering how i regulate flow into my FT and Growbed so that my autosiphon can be tuned? Im using SLO from fish tank to GB and i dont see as i have any choice but to put a valve between the pump and the FT if i need to slow flow rate. going from sump, to FT, to GB and pumping from sump obviously.
It really depends on your pump. Mag-drive pumps deal with this better than other pumps. You just have to be careful you don't burn out your motor by restricting that flow. An alternative is to split the line with a return to the sump and you restrict THAT flow instead, full open is half the normal rate to the fish tank, full closed is the normal rate to the fish tank. Its safer for your motor and can allow you to install a second valve so you can turn off to the FT entirely while you work.
Why waste that water flow? Like Christian said, you could "t" off your plumbing with a valve and and run some of that water back to your sump tank via an aerating bar for some extra aeration.
I have five ball valves on my system, the water goes where I want it to go, two grow beds with flood and drain plus a pressure relief valve back into the sump tank. Its best not to over burden your pump but at the same time make it work for you.
I haven't had any problems with my pump and using the valves, plus it allows me to "balance" my bell siphons. The are very handy for making adjustments to the water flow. I've had my system up and running for almost 2 years.