Sounds like a reasonable pump for the job Matt, many pumps give details about flow at different heads on their box, or in the instructions, or perhaps from a website etc.... I did a quick web search but found nothing on that name of pump..
So far as pipe sizes go, I would try and make the drain pipe larger if possible than all other pipe sizesdelivery and return pipes are under pressure from the pump, the water is forced through the pipe, but drain lines from the beds are only under the pressure of gravity.. I guess it depends on your ultimate design, but you need to be sure that the water flowing out of a bed can always deal with the rate of water filling the bed so that no flooding can happen.
The gravel I use has just under 20% air spaces, hence the water will those air spaces. This would mean that in 2 cubic metres of growbed it would be about 400litres, however, the water is only ever filled to about 30mm (an inch) below the gravel surface of the growbed , so remove about 10% off that. So about 350 litres, about 1/3 of your water when you have a 2:1 grow bed to fish tank ratio.....
I don't know about manure in the tank, maybe compost or worm castings, some of the fishy people here may have more of an idea...
