Easiest way to introduce new fish into a tank with guppies?

To introduce a new fish into your aquarium, begin by floating the bag on top of the water for about 15 minutes. After 15 minutes open the bag and roll down the sides. Pour a small portion of your tanks water into the bag. Wait another 15 minutes and then net your new fish from the bag and release them into the aquarium. Do not add the stores water into your aquarium.

A 5.5 gallon is too small for an algae eater. The smallest sucker fish is the otocinclus at 1.5 inches, but they need to be kept in groups, and require more space.

I honestly wouldn't add anything new to the tank.

In a tank that small, you have plenty of fish and don't need to add any more.
Aside from the "giving them room" problem, the more fish you have in the tank the faster the water quality will diminish. you'll have to do more frequent water changes and monitor ammonia/nitrites/nitrates more often as well.
Bad water quality will lead to disease and you might end up with no fish. Get a larger tank before getting more fish

If you have an algae problem, only get an apple, or mystery, snail. It will take care of the algae. They don't reproduce like the hitchhiking snails that enter your tank with new plants. But if you ever have to treat the tank for ich, take the snail out first. That medication will kill it.
Obviously, you know about acclimating the temps of their bag water before putting them in the tank.
The best way to introduce them is at feeding time while the other fish are distracted, or at lights out. Both ways will cut down on the nipping/territorial problems, but might not completely keep it from happening

In a tank that small, you have plenty of fish and don't need to add any more.
Aside from the "giving them room" problem, the more fish you have in the tank the faster the water quality will diminish. you'll have to do more frequent water changes and monitor ammonia/nitrites/nitrates more often as well.
Bad water quality will lead to disease and you might end up with no fish. Get a larger tank before getting more fish

If you have an algae problem, only get an apple, or mystery, snail. It will take care of the algae. They don't reproduce like the hitchhiking snails that enter your tank with new plants. But if you ever have to treat the tank for ich, take the snail out first. That medication will kill it.
Obviously, you know about acclimating the temps of their bag water before putting them in the tank.
The best way to introduce them is at feeding time while the other fish are distracted, or at lights out. Both ways will cut down on the nipping/territorial problems, but might not completely keep it from happening

I would say only one more fish because the general rule for a tank is one inch to one gallon of water. To introduce the fish, place the still filled bag you get it in at the top of the tank in the water. This give you the chance to catch any aggressive behavior and also for the water in the bag adjusts to the temp in the tank so its not a huge shock. Since your tank is so small, I would go with the snail. They do the job and they don't outgrow the tank. They also require little to no maintenance.