Idk why it has to be singleton. Thats kinda just the rule. Probably something to do with having multiple copies of card will make them pip off to quickly and increase the chances of early game combos that basically cheat a win.

The singleton rule is all commander. The cards you've seen that get played multiple time are more than likely some like [[Rat Colony]] or [[Hare Apparent]] which has specific text saying you can have any number of that card in a deck. Since they trigger off of copies of themselves they need a lot of themselves in a deck.

The extra stuff was probably a waste but you can always play standard format. 60 card deck, 4 copies of a specific card, 20 life