Most Plecos need large tank (Ex: Common Plecos need at LEAST a 75 gallon [about 284 litres] tank as they can grow to about 12 inches long) Bristlenoses on the other hand only grow up to about 5 or 6 inches and need a 20-30 gallons [about 75-113.5 litres] minimum for them. The smallest Pleco grows to about 2 inches and is called the Gold Spot Dwarf Pleco. It is good for a 10-15 gallon [about 38-57 litres] tank.
Instead of Plecos you could try other bottom dwelling fish such as Corydora Catfish, Amano Shrimp, Otocinclus Catfish, or Kuhli Loaches. These all a stay fairly small and should fish in your tank.