Hi Donny,
Take everything a shop tells you with a huge pinch of salt. Too many of them haven't a clue what they are talking about but they are extremely good at sounding convincing. There are some that instruct the staff to make things up to make a sale. For instance, I heard someone in one shop telling a customer to add a bottle of bacteria to a brand new tank and also add all the fish he wanted at the same time or the bottled bacteria wouldn't work. I went home and googled the product in question, and it said to add a few fish at first, wait a couple of weeks and add more a few at a time. Not exactly what the shop was telling customers!
Regarding a fish-in cycle, that's right. Get a few fish, measure both ammonia and nitrite daily (twice a day is better) and do water changes every time you see either/or ammonia and nitrite. A lot of people find it helpful to make notes of the readings so that one day when you say to yourself 'I haven't had any ammonia or nitrite for a few days even though I haven't done any water changes' you can look at your notes and see how long it's been
When it's been a week, the filter is cycled for that number of fish. It could take anywhere from a month to three months to reach that stage, you just have to be patient. That's one thing fishkeeping teaches us, because these are living things (including the filter bacteria) they go at their own speed which may be slower than we'd like. Don't forget you can only get more fish slowly. If you had four fish, you could add maybe 2 more once the filter is cycled, then a week or two later, two or three more.
If your son's tank is square, that sounds like a biorb life. The main problem with stocking biorbs of any shape is the fact that the rocks on the bottom are the main part of the filter where the bacteria live and they are quite rough, so that means no corydoras or loaches. These fish like to root around on the bottom for food but they have barbels which can get cut on rough stones which allows infections in.
There could also be a problem with using one as a quarantine - it is usual to move the quarantine's filter into the main tank to keep the bacteria alive when the tank isn't in use, but you can't do that with a biorb's filter. The alternative would be to keep fish in it all the time and swap them over to the main tank when you want to use it for quarantine.
But for your 80 litre, what are the dimensions, that would give us a better idea of fish for it. Some tanks are long and shallow, others are tall and narrow. And how far away would you sepnd most of your time viewing it from? It's no good recommending tiny fish if you woud be too far away to see them
And it would also be useful to know the pH of your water and whether it is hard or soft (eg does the kettle fur up or not). It is much easier to get fish to suit your water rather than try to make the water suit the fish.