You'll have to do 50% weekly water changes to keep up with the bioload these guys produce, I know if seems like a lot, but they definately need it. For filtration, I'd highly recommend an AquaClear filter at least 2x your tank size (for a 30gallon tank use the AquaClear 70 filter) And don't use carbon, it isn't very good for puffers. Instead what I go with is a layer of sponge, a layer of whatever those white rocks that come with the filter are (I can't remember what they're called at the moment) and then another layer of sponge. I've never heard of using a skimmer on a freshwater tank, and I don't think it'd do much.
With tank size, long would be better than tall, but you could really go either way.
Stocking:
You definately don't want to over stock a puffer tank, I guarantee you it'll end in disaster with either your fish killing each other or the ammonia getting too high.
10gallon: 1male dwarf puffer and 3females
15gallon: 1male dwarf puffer and 4-5females OR 1Figure Eight Puffer
20gallon: about 7 dwarf puffers OR 1 Common Puffer OR 1 Red Tailed Redeye Puffer OR 1 Bronze Puffer OR 1Figure Eight
30gallon: about 10 Dwarf puffers OR 1 Common Puffer OR 1 Abei Puffer OR 1Red Tailed Redeye Puffer OR 1 Bronze puffer OR 2 Figure Eight Puffers, OR 1 Green Spotted Puffer OR 1 Ceylon Puffer
55-60gallon: 4 Figure Eight Puffers OR 2 Common Puffers OR 2 Abei Puffers OR 2 Red Tailed Redeye Puffer OR 2 Bronze Puffers OR 2 Green Spotted Puffers OR 2 Ceylon Puffers
Remember though that puffer species cannot be mixed together in a tank, Green Spotted Puffers grow much larger than F8's and will eventually need a very high-end brackish tank, or a full marine tank, while F8's do best at a low-end brackish s.g. And a Ceylon Puffer will outgrow a Green Spotted Puffer.
Both Ceylon Puffers and Green Spotted Puffers do best in full marine conditions when full grown, if you're looking for something that'll stay in Brackish water, I'd go with the F8 puffer.
Most of the puffers I named off are Freshwater, there are very few common Brackish puffers.
The PufferPedia is a good resource when looking for puffers
http://www.thepufferforum.com/forum/ug.php/v/PufferPedia/
If you want to add multiples of puffers to a tank you need to double the tank size. So a puffer that needs 30gallons would need 60 when a second puffer is added and 90 when a third is added.