So its 18 inch by 12 inch? How high is it? then someone can work out how many gallons it holds. It sounds quite small so i would avoid the goldfish and weather loach as they can get a foot long. You could probably keep about 6 zebras in it depending how many gallons it holds (if you mean zebra danios which only grow a couple of inches.) And a few snails. The loaches like to live in groups so give them a miss. If you already have a weather loach you could get a bigger tank (at least 3 foot long) and get it some friends. The recommended tank size for goldfish is 20 gallons for the first fish then 10 gallons for each additional fish.
Emma
edit: another fish which stay small and could live with danios is the white cloud mountain minnow.
Your tank's volume is approximately 11.22 U.S. gallons, which is approximately 43.65 liters. So too small for the goldies or loaches. stick with the danios and white clouds until you can get a bigger tank. These small fish go by the general rule of one inch of adult fish per gallon so you could have 6 of either in a tank that size.