Ok, first let me address the cardinals. The sooner you can post test results for ammonia, nitrite, nitrate, and especially pH, the better... if you can add Gh & Kh in there, those will be important too. I am hoping you purchased liquid test kits... can you let us know what type they are? (brand name)
Cardinals require softer water than many of the other commonly found tetra species, and are a more suitable mix with fish such as discus. Cardinals also thrive at much higher temps that most other tetras require or more than some can handle... in the 82 - 86 degree range. It is not a common thing to acclimate cardinals to harder water successfully.
Personally, if it were up to me, the remaining cardinals would go back to the store and would be replaced with fish that are more suitable to the environment you already have established for them. I would also strongly suggest testing the water before adding any other fish, so you know what type of enviornment you already have to work with. If the water is soft, how soft... if its hard, how hard... and work within those limits and find fish that can thrive within those limits.
In regards to the pleco... if you provide plenty of food there is no need for the fish to scavenge for food, as in algae in the tank. How much algae growth do you currently have in this tank? If there is a large amount of algae on rocks, glass, etc. then my next quesiton would be why is there such heavy algae growth in a tank with very few fish and a pleco species known to have a heavy appetite? This answer is also likely going to show itself in your water chemistry, and possibly in your lighting. Another question to ask in this sort of situation, what type of algae are we talking about? Is it slimey and soft, covers everything in a sort of blanket as it grows... is it hard and covering the tank walls, seen as a green "sheen" on the glass and possibly rocks... or is it hariy/fuzzy.. growing in various places, specific places, etc?
Please remember that an algae eating fish will not eat all species of algae, and this applies to any algae eating animal. Each species of animal has its food preferences, which are usually limited to a few specific species of algae. Once I have a better description of the algae growth in your tank, then I can better suggest if you should slow down the feeding of your pleco. Until we identify what type of algae you are dealing with, I would continue to feed it, just be sure to limit the amount of food. A very small pleco will not eat a large amount at a time. If offered a large amount, whatever that fish doesn't eat within the first 1 - 2 minutes will begin to break down into the water and can pollute the water, add to an already existing problem, etc.
Can you post any photos of this tank? Visuals can often lend a lot more light to a situation for those of us who cannot see it first hand. Please try to get the clearest photos possible. Blurry pictures make it difficult and sometimes impossible to determine a problem.
Sorry I can't be of more help yet, but the one thing I will strongly urge is to not add anymore new fish until we first have a better understanding of your water chemistry. Your water chemistry in that given tank should be the overall determining factor in what fish are options for you to keep in there.