I'd be worried about putting a 3 inch goldfish with the zebra danios. Goldfish and tropical fish are not the best tank mates. For one, they thrive in different water conditions (goldfish like colder water, higher pH). The other thing is that the goldfish may start to look at the danios as a food source. If you want to put the pleco in there you can. Just be careful. I'd wait for a few weeks of water testing with the danios present. When you see 0 ammonia, 0 nitrites and about 20 ppm of nitrates then you'll have a tank that is safe for the danios. At that point I would add the pleco and start monitoring the water again. The pleco should give you an ammonia spike within a week of it's introduction (maybe longer). It won't be a huge spike but one that can be measured. Then repeat the process of watching for 0 ammonia, 0 nitrites and 20 ppm nitrates. This will be the best process for any future additions you make to this and any other tanks you may own. You never want to add a lot of fish at one time as their waste will overwhelm you bacteria colony. The key will be patience. By slowing adding the fish you want to your tank you will ensure their health.