There are several problems I can see with putting an Axolotl and a Pleco together.
Space. Axolotls generally get to be about 12-14 inches in length, while common plecos will get to be about 2 feet long when they're fully grown. A common pleco itself needs a minimum of a 40 gallon(about 150 litre) aquarium.
Temperature. Axolotls are naturally from coldwater ...
It is generally not recommended to keep any type of fish with Axolotls as they can nip at their gills or otherwise stress the axolotl.
Plecos are definitely not compatible with axolotls for several reasons. One is the difference in needs for ideal living environment. Plecos thrive at warmer temperatures than are safe for an axolotl. The most important ...
As far as the tank goes, sirens can grow up to 24 inches so get at least 3 times their length as the size of the tank.
They live in shallow water so you don't need to fill the entire tank.
Temperature isn't that important as they are used to living in shallow pools of water.
Get the finest gravel you can get because they like to burrow. Ideally you would ...
I keep a golden bristle nosed pleco in my tank along with two Axolotls. They have lived together for almost a year now with no problems hardly ever. I would only keep these types of Plecos with Axolotls because they are a lot shyer than most plecos and they live in colder conditions than all other plecos. Other plecos aren’t great though.