Our practice in the last 20+ years is to make sure the pipe sizing is correct based on the application.

We customarily use either 3" or 2.5" on all suction/inlet lines such as surface skimmers, floor drains or strip drains. The idea is keep the water velocity at a lower rate to allow for better supply to pumps when operating. Smaller piping results in high water velocity and increased suction. This is not necessarily good as there is a danger of too much suction and it also causes the pump to cavitate - starve for water supply. The illustration I give my clients is, it's like sucking water through a cocktail straw. The supply is needed to perform properly.

I don't have eyes on you set up, but a typical residential pool as yours is described, we will use 2.5" on the surface skimmers and 3" on strip drains. Some builders use floor drains. Sometimes we use 3" on all inlet lines. It depends on the distance from the pool to the equipment set and the volume of the pool which determines the proper turnover rate.

We use a separate pump for the jet circulation. Piping for this is always 3" minimum on both the suction and the return. A looped manifold is created to provide even water distribution at each jet.

It is not advisable to use the filtration pump as the source for the jets.

A pool/spa combo shares the filter pump and has a separate pump for jets-only. You have the option to add an air blower or use ambient air to create bubbles.

Using the largest filter is the best idea. The VS pump is fine. Just be sure the flow rate is set properly to fire the heater when calling for heat. Too much water flow will eventually damage the heater and conversely to little can cause damage as well.

The spa may heat in 15-20 minutes based on a 20-25 temp swing. Starting temps and ambient temps impact heating time. Site conditions such as wind make a difference. Consider an automatic pool cover. It is an additional cost but it helps maintain temps, saves on chems and creates a safer environment at your pool side.

At 125' of poly gas pipe, which includes all fittings (elbows/90's, etc...) you will get a delivery of

410, 000 BTU's. To me, that;s too close for the heater you plan to use.

Your best bet is to install a 2" line which can deliver 780,000 BTU's at 125'. All the fittings and pipe reductions at the meter and the heater have an an impact on fuel delivery.

I also recommend a total gas load calculation is performed to see what presently your house requires, and then factor the pool heater. The gas company can do this calculation and recommend the proper gas meter if needed.

We run 2" gas pipe for all residential jobs. Some special cases we have run 3" if sharing a common line with say a giant house generator.

Not a fan of any suction side pool cleaners. We prefer pressure side cleaners that utilize a separate booster pump.

Hope this information helps you.