I am an African Cichlid person, but not an expert yet. That being said, there are a few basics you should know.
1) Never mix cichlids from different lakes or different countries. If you have a Cichlid from Lake Malawi, never add a cichlid from Lake Victoria.
If your Cichlids are from South or Central America never add an African.
2) Africans tend to be aggressive towards each other. To keep them happy you need to have 3-4 females for every male. If your Blue Cobalt is male then he will need at least 3 female Blue Cobalt's.
3) Your Arautus is HIGHLY aggressive, although beautiful. But, not only will the males kill each other, the females will also kill each other.
They actually require a minimum 75g tank, the 55g is to small.
An interesting side note on the Arautus is, the females are Bright Yellow while the male are brown and grey, very rare among fish. The male to female ratio needs to be a bit higher with these guys, a 1:5 will do. When the males are young they look the same as the females. This protects them from being killed by the larger males. And from what I've read about them, if there is an imbalance of males to females, the Arautus can change gender to repair the imbalance. In nature at least, I'm not sure if they do this in captivity.
The Cichlids from Lake Malawi don't have mating pairs, each male has a small harem to call his own, they're polygamists.
Now, about those Common Plecos of yours. At full growth they are an average of 22-24" long and require a minimum of 125g tank. To raise them in a 55g tank will cause a condition called stunting. It deforms their spine and internal organs, along with dramatically shortening their lives.
They are great pets and if you have the room, are well worth having.
We were given one that we had for years. He would come up to the surface of the tank when we walked in, so that we would pet him, just like a puppy. He was hand fed, and insanely friendly. But they put out about 5 times the amount you feed them. Basically they are a constant poop machine, but a friendly poop machine.
There are countless smaller Plecos that do great in a 55g tank. I have a Regular Bushynose Pleco, he's 5" long and full grown. I also have 2 very cool Green Dragon Plecos, that will be 5" when they grow up. And 2 Long Fin Bushynose Albino Plecos, who will also be 5" long as adults. They too have a large bio-load for their size but it's nothing when compared to the bio-load of a Common Pleco.
I don't know anything about the cats, other than they need to be in a bigger tank.
As is, none of your fish should be in the same tank and only your Cobalt Blue Cichlid can thrive in a 55g tank. The others should find a new home.
The Mbunas are great fish. There are many beautiful colors and they are active interesting fish. And as long as you have the correct male:female ratio,their shouldn't be a lot of fighting.
But like any other pet there can be personality conflicts between fish. When that occurs, you will usually have to get rid of one of them.
I had a Red Zebra early on that turned out to be a psycho serial killer. He killed both males and females whenever he could. My LFS took him back and I never got another RZ, again.
I know it isn't easy to re-home fish, especially ones that get to be two feet long. But it's torture for them if you don't.
First, check with your LFS that isn't a chain. They are, in general, more willing to take them off your hands than a big chain PS. You can also put them on craigslist for re-homing. Some area's still have those free Penny Saver papers, if you do, they are also a good place to put an add for re-homing.
Best of luck to you.
