If you buy a pond pump rated larger than 9000 gph you can always put a t piece and diverter valve on the pipe work to send some of the water straight back to the pond and some through the filter. This also helps when cleaning the filter as all the water can be diverted to go straight back to the pond whilst you clean the filter (unplugging the uv first of course).
Most pond pumps come with variable sized adapters to fit different sized pipe work so you should have no trouble matching the pipe work of the filter and hence that hose size to a decent pond pump. Only when you do connect it, first cut off any smaller connections as this will restrict water flow.
Always use the biggest fitting on the filter and pump and hence the biggest pipe you can as this allows better flow and doesn’t restrict the rating of the pump.
Note however that you need to rate the filter and the pump to the pond you have or will be making and the stocking level you have/ want. So if you have a 5000 g pond then a 9000g filter would be fine but if you had it heavily stocked with koi then you would need a bigger filter and hence bigger pump.
Note that you also need to take loss into consideration when rating the pump. If it has to pump up to the top of a waterfall then the flow rate will be greatly reduced. So a 9000g pump may only be pumping 4000g for example at the top of the waterfall where you have your filter, so you could have bought a bigger pump! The pumps should come with a chart showing flow rate at water height (so how high you need to pump above the level of the pond) to help you chose the right one.
