It's great your boss knows of and encourages cycling and filtration , I think you've got a better chance than some of convincing him about the betta tank setups.
You could also try explaining about how goldfish need really large tanks, and why the small setups are useless for them ( which he probably already knows but may need reminding of ) You could propose that the store gradually decrease the number of young small goldfish they buy in, as this fools people into thinking they stay small,
and perhaps have a large show tank with some fully grown adult goldfish to illustrate just why those small setups are no good for them to show customers how a goldfish should be kept, how much work and expense it actually is. (due to large tank and large external filtration ) and why they are not so suited for a beginner or child as people might think.
Then you could propose that the goldfish setups be redone as betta setups ( 5 gal ones anyway ) in the way you've described, and if people want an alternative to goldfish for a smallish tank ( many people have something in the regions of 10-20 gallons ) then you could offer them a heater and some
Gold barbs as an easy to care for fish that won't need to be upgraded to a huge tank, won't overload the filter and can be kept in groups safely in an average sized tank.
They look enough like common goldfish in shape and colour to satisfy most people , will eat pretty much anything you care to throw at them are a relatively hardy fish for a beginner to start with.
Your boss should be able to see the sense in that, and the store wills till make money from the sales of barbs and bettas.
As for tanks smaller than 5 gals, why not sell them as shrimp setups if the store's main small tank customer is kids? 3 gal kits ( for example) with a little sponge filter , some gravel , dechlor and a plant plus a pack of shrimp food and a basic care sheet, then all they need is shrimp. Most kids like bugs or weird critters, so a shrimp colony is ideal and easy for a child to look after with supervision.
I bought a 3 gal tank in a shop selling mostly toys last week, it came with a sponge filter, plant, fishfood, net , laminated plant background sheet and airpump to run the filter. It was marketed as a goldfish tank
but will be perfect as a shrimp tank and is almost the same as the setup I described above.
That tank was on sale at £12.99, but it's original price was £20 and there were only 2 left in the whole store so people have been buying them at the full price.
It's worth considering.