It is generally not a good idea to mix species, for a lot of reasons. Salamanders (and therefore axolotls, too) are carnivorous predators and so whether they are aggressive/territorial or not, they may still try to eat each other. Different types of (mating) behavior may cause problems. Differences in size would be an issue, too, as I believe most salamanders are much smaller than an axolotl. In some cases you may also have incidence of the two species breeding together, sullying the special strains. Et cetera.
I think that if the paludarium is big enough to give the axolotl the room it needs to move and grow and still hold the plants you want, it should not be a problem. I do not think that the axolotl will try to walk up the plants and out of the paludarium. I am sure that there may be smaller, fully aquatic newts that would also work well and not need as much space as an axolotl, but someone else would have to suggest a species because I don't know anything about newts.
If you set up a paludarium, with or without an axolotl, you will give us photos, right?
-Eva
-Eva