Don't put any more goldfish in a tank this small. Just one will outgrow this tank soon, no matter what you may have read.
Fancy goldfish are very deep bodied fish and get to 8" - they also produce tons of waste, not just visible like poop but I'm talking direct ammonia production after nibbling flakes, pellets, etc. so that the water will tend to get slightly yellowish right after feeding. A filter rated for 2x the tank size is recommended to help cope.
Any visible algae will get eaten by the pleco, but keep in mind common plecos get over a foot long, need a heater in the winter (temps no more than 65 degrees will be tolerated) and there are many reports of sucking fish like plecos and Chinese algae eaters sucking on goldfish. If you keep your pleco well fed (a daily algae wafer and veggies, say) it may not do this, but you never know. Overall, it's best not to use fish to "clean" - nerites snails are fine, though. But a balanced tank does not have many algae issues.
If you feel like your tank looks empty with just one goldfish, you can try white cloud mountain minnows or zebra danios. But no more goldfish unless you can consider upgrading to a 40-55 gallon tank.