I wonder which one is better: a 20 gallons long or simply a 20 gallons. does it have a difference for the fishes? more place in a long?
, a 20L is better than a 20T, simply because there is more surface area. More surface area means there is more usable space for the fish to swim, better gas exchange etc..
In your climate, I would go with 100W, I recommend 3-5 watts per gallon on heaters, and that far north...I would go with the max. However, rather than buy 1 large heater, I prefer 2 undersized heaters, because if one stops working, there is still another to provide heat, even if not enough, and if one sticks on, the other will shut off and there won't be enough to boil your fish
For a 20g aquarium, I use a remora skimmer by Aqua-C, I also used a bak pak 2R and was happy with the results. There is an article in the articles section on skimmers and what they do.
As for lights...how much light depends on what your gonna do with the tank, Reef, FOWLR
How many fish...depends on the species. I have three small fish in my 20L and it is probably overcrowded.
I'm not sure what you mean by reef package.