In general, yes. When you're simulating a traditional physical gaming device (a pair of dice, deck of cards, roulette wheel, lotto balls, coin flip), it must match the real device.

From what I recall, card games are a very special animal in the eyes of the gaming board. When you simulate dealing from a deck, there must be a deck of 52 cards in memory. You cannot just pick random numbers from 1 to 52. I believe every hand must have a freshly shuffled "deck" as well.

The way video poker machines manipulate their odds is through the payout. Watch veteran players when they approach a machine. They'll note that some machines offer slightly different payouts for lower wins, and that's where the odds grind out. You can't change the probability of a full house, but you can change how much is paid back.

Slot reels are a loophole. Because you can't see the entire reel, how do you know it only has 20 stops? What if it has 20,000? This concept (known as the Telnaes patent, US #4448419) enabled virtual reels and much much larger payouts than the 8000:1 jackpots before this invention.

Later on, the invention of bonus games and etc kind of ripped up the virtual reel idea. But the math operates on this principle as far as the PAR sheets are concerned (i.e a certain set of stops on the reels starts a bonus game with another known set of odds).

Virtual slots definitely do not have to simulate the same reel size as their physical counterpart. I've only worked in online gambling, but almost ever slot game I saw had larger reelsets. Some don't even simulate physical reels, but instead just use a probabilistic model to decide what appears.