Skimmer Box & Jets
Skimmer boxes come in a variety of sizes and the rule is generally one per 50,000 lts of water. Just keep in mind that if you are going to run a pool cleaner you will need to block off one of the skimmers anyway. So unless you have a really big pump there is no need for 2 skimmers. If you plan on a 100,000lt pool then make the pump big enough to run the pool cleaner from one and still have flow left to surface skim from the other skimmer box.
Always plumb suction lines in 50 mm pipe and return to pool lines in 40mm pipe. This gives the pump a good head of water. If you have 2 skimmer boxes plumb them both in 50 mm pipe and don't join them until immediately before the pump. This gives even and maximum flow to the pump.
Return jets are available in a wide range of styles and colours. It's simply a personal preference. They should however be strategically placed around the pool for maximum circulation. If you have "dead spots" in the pool then this is where the algae will breed first.
These come with the pool as part of the package and will be what the manufacturer recommends for that size pool.
Something to look out for though is the size of the skimmer basket. It should be at least 15cm edge to edge and at least 10cm deep.
All in ground pool Skimmer Boxes should come with a vacuum plate so that you can connect a vacuum hose to your skimmer box. Some Skimmer Boxes use the Weir Door (floating flap at the front of the box) that has a hole and slide which folds down over the basket.