10 gallonhat, eruna is right about the guppy. He/she is a warm water fish, while the goldfish are cooler water. Also the guppy does much better with salt, which is a no, no for gold fish. Just a little info on your gold fish. There are 3 groups of gold fish as follows:

Group 1-Pond comets/Shubunkin. Two lobes on tail with one being above the other. This group swims faster than groups 2 & 3, and will eat most of the food. They will usually nip the fins on groups 2 & 3.

Group 2-Fancy gold fish. Two lobes on tail fins arranged horizontally with one to the left and one to the right. Includes, Fantails, Black Moors, and Orandas.
Swim much faster than group 3. Will eat most of the food, and will usally nip on the fins of group 3.

Group 3-Very Fancy. Ranchu, Lion Head, Bubble Eye, Celestials. This group has very limited swimming ability. Very slow movers. Will be nipped by group 1 & 2.

All groups should be kept seperately. They should only be kept with others mentioned in their group. All the goldfish in these three groups are the same fish species, Carassius Auratus, and could possibly interbreed.

Koi on the other hand are a species known as Cyprinus Carpio. They will certainly grow too big, swim too fast, and will eat all the food. They will nip all gold fish from groups 2 & 3. They can get along with fish from group 1 for a while, but not always. Shubunkin or pond fish do get large, but are not koi.

Hope this helps you some on your decission to keep goldfish. Please take a look at this web site. aquariumfish.net. This sight is run by the Bailey brothers who have been in the fish business for 35 years. They have tons of information, and some super fish for sale.