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 streams/lakes, while freshwater fish kept as pets are generally from warm/tropical streams/lakes. The temperature range for axolotls is about 60-65°F (16°C-18°C), while the temperature range for plecos is about 72°F-82°F (23°C-28°C).
Ideally, you could find a coldwater fish and keep them with the axolotl, but the main problem with keeping things with axolotls is that they try to eat everything. I know some people who will feed guppies to their axolotls. So my worry is that if the pleco isn't the same size as the axolotl to start with, the axolotl will try to eat it. Plecos have bones in their fins that would catch in the axolotl's throat.
Here's an aggregated list of disasters involving people mixing species. One of which is a pleco and an axolotl. These are the worst case scenarios, but it's all there to show that these things do happen.
Which brings up my next worry, that plecos are notorious for chewing on everything. They commonly suck the slime coating off of other fish, causing them to get sick. I'd be worried about other fish as well, I'd be willing to bet any fish would nibble at the axolotl's gills.
All in all, the best companion for an axolotl is another axolotl. I don't think the risk of the axolotl choking on a fish, or a fish chewing on the axolotl's gills, is worth any perceived friendship.
That being said, I know people who keep apple snails in their axolotl tanks to take care of the algae, but that is because the snails can be big enough that the axolotl's can't eat them. You have to get one that's as big as the axolotl's head, or you'll be pulling it out of it's mouth later.
I have heard that ghost shrimp are ignored by axolotl's pretty regularly, so if you're looking to have something in your tank other than axolotls, that would be my suggestion. Worst case scenario is that they get eaten.
Axolotls Water Requirements