I use the Shark filter in a half-filled 20 gallon tank, and it has not caused any problems. The water is actually very clean, but I am currently using it on the side of the tank now and not the bottom. Water stays around room temp at all times (but I wish it was lower!
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But yes, a two gallon may be good to keep on hand for your feeding stock, or as a quarantine tank which would need frequent water changes anyway. Ten gallon is the smallest I would recommend, as a five (even with only one C. orientalis or some other small species) will get dirty and dangerous quickly. The water also tends to be warmer the smaller the tank is.
Another important thing is not leaving your tank lighting on too long (if you decide to light the tank). Buying a timer is relatively inexpensive and can be very important if you work weird hours or might forget to keep your newt on a regular schedule. You can get them at Home Depot.
Don't listen to anything the people at the pet stores say because they generally have no amphibian knowledge. It is also a good idea to familiarize yourself with several exotic vets and determine which has the most training or knowledge on amphibians. Most people (as much as this saddens me) treat newts and sallies as "disposable" pets, but if you care about your animal it may even be a good idea once you find a good vet to take it in to have it checked out. Shipping conditions for these animals are often insufficient and can create health problems that you won't find out about until too late.
Caudate Central (part of this site, and my apologies for mis-quoting an url for it in another post) has a ton of care information that you will find invaluable and most likely won't locate anywhere else.
One neat idea for setting up an island (which must be done well before "relocating" your newt) is to buy Aquarium grade sealer and cut some plexiglass to be mounted just above water level in your tank. You can set up whichever substrate you prefer for this area, and take a small piece of driftwood or a rock to provide a ramp up to this area. It is space efficient without creating a "dirty area" that a rock grade would. If you do this, make sure you rinse the tank thoroughly before setting it up with VERY hot water, and always remember to begin the nitrogen cycle before introducing an animal to a new environment.