What kind of "sucker fish " ??? You will probably need to rehome him, as a 5g is really tiny for that many fish. Plus if he's a Pleco he will get BIG.
I'm really sorry to say you should also rehome 2 of the Platies. You are overstocked.
Platies get 2" for males and females can get 2.5"
That is partly why your tank is dirty. Too many fish, too much poop
So options are:
Get a larger tank, a 20g would be better for Platies IMHO
. Keep the small tank, so you can cycle the large one faster, using filter media from the 5g
Rehome all but 2 of the Platies and only keep the same sex. Babies could crash your tank.
Rehome all the fish and restart the tank with a better plan. Chili Rasboras and some live plants would look great
or we can help you set up a Red Cherry Shrimp tank ( my favorite ).
We've all been in your shoes. We've all had to learn better fish care. This is a good place to go forward with a healthy tank and thriving fish.