The general rule is minimum 10 gallons per axolotl so it is suitable for one axolotl. Your tank is about the same size as my quarantine tank, which I currently have one axolotl in.
I looked up that filter and it appears to be an internal one, which can be okay but they do release a lot of heat into the water. I would recommend a hang on back filter and small sponge filter. Sponge filters are pretty cheap and will attach to the air pump you already have. You won't need the air stone as the filter will oxygenate the water enough.
What kind of ornaments are they? You don't really need them for axolotls, you're better off using the space for hides. Axolotls aren't fond of light so need some nice dark spaces to hide, even if you don't have a tank light. I use ceramic pots placed on their side, super cheap and easy. You can buy hide ornaments, but watch that they don't have anything sharp. Most of the ones I have seen are sharp inside, which can injure axolotls.
Plastic plants are great to use, it provides extra shade and some like to sit on them.
What substrate do you have in there already? You want either big stones (bigger than an adult axolotls head. I don't like these because they trap debris and poop!), sand or bare bottom. I find sand the easiest to clean, I have bare bottom in my quarantine tank and it's a pain to keep clean.
Depending on where you live and what temperature the water will naturally sit at, you may need an aquarium fan or chiller. You want the water under 20 degrees.
Sorry for the paragraphs! Hope this info helps. Please ask if you have more questions
