PLEASE research before buying.
Axolotls require very specific care, and judging by your questions you have no clue about that care.
NO, they cannot be converted to brackish. They are strict COOL water freshwater fish. Their absolute max temp is about 73F.
They should only be kept on sand. As stated gravel can be ingested and cause impaction. Bare bottom gives them nothing to grip which stresses them. It can also cause sores to develop on their toes.
The only thing I have found is even close to an option as a tankmate are fancy goldfish. Fancy goldfish are too slow to nip at axolotls' gills without getting nipped back. They usually get nipped by the axolotls once and then learn to avoid them, then living together without issues. However, this is not always the case and it requires a larger tank.
They should be fed high quality pelleted foods. The standard is trout or salmon pellets, but New Life Spectrum worked at least as well for my axolotls. No need for big meaty foods, stick to high quality pellets.
High water quality is just as essential for them as it is for fish. Frequent large water changes to keep the nitrate concentration under 20ppm are required. Aeration is very important as well, especially at the higher end of their temp range. Too much flow from power filters can stress them, as will excessive light without hides. I used PVC pipe connectors, they loved them. They provide great hides and are very smooth so they don't scrape themselves.