What size aquarium should I use for 2 Koi fish?
500 gallons minimum. An alternative would be two interestingly colored comets. They have the same general body type as koi and longer fins than commons, but could be kept in a 125 gallon tank. Just don't pick the first two you see, take your time and be choosy. Since they aren't really aggressive, you don't even need to pick two from the same place, just find two that really appeal to you.
Can I introduce multiple fish into an aquarium at the same time?
you can add different species of fish at once, but 5 fish is an awefull lot to add at once. and ottos are very sensative to water change. i would never try to keep them in less then 10 gallons. what size tank do you have? both ottos and glofish are schooling fish, there color and behavior will be much better if you keep them in proper schools, 5 should be the minimun for zebras, and 4 is a bare minimum for ottos. lonely schooling fish will die prematurely from stress. your better off choosing one species or the other, but it sounds like you need a bigger tank, the bigger the tank, the easier to care for.
(Petsmart employees?) Should I just buy an Aquarium Kit...?
The reason why I usually don't like the kits and buy everything individually is because the kits will usually come with the cheapest equipment. I will spend the little extra money and get the equipment I want first instead of getting the kits and upgrading later. A standard 20 gallon tank is usually 24 x 12½ x 17 Tall. MFK
What's the average aquarium size?
Well I'd recommend looking at local fish stores as well as online for the prices of tanks, equipment, the fish you want, food and that. I wouldn't recommend getting fish unless your willing to spend a few hundred dollars ($200+). It adds up for the light, tank, stand, substrate, fish, fish food, filter, replacement filter cartridges, dechlorinator, heater, thermometer