Hi Bruce,
The filter usually provides more than enough aeoration to the water so you don't 'need' an air pump. It's really a matter of personal choice whether to have one or not. Some axolotls enjoy playing with the airstones and some find it stressful. If the axolotl is displaying any signs of stress with the airstone remove it as stress can lead to illness.
The only fish that can be kept safely with axies is feeder fish such as guppies and white cloud minnows (fish intended for food). If you love your fish, don't put it in with the axolotl. The axie lives by the axie code: "If it's smaller than me, moving by me, it just must be food!".
Larger fish can make an axolotl's life miserable by nipping the axolotl's gills and adding to water quality issues. Not many fish can tolerate the cold temperature which an axie needs to survive which is another issue. If the axolotl snaps at a fish that is too big, the fish can get stuck in the axolotl's mouth. Most importantly, some fish have natural defences such as spines which can kill an axolotl when the axie tries to eat it.
I would recommend you read through this 'mixing disasters' article:
http://www.caudata.org/cc/articles/Mixing_disasters.shtml
If you haven't already found this part of the site, it gives all the information you need for the care of axolotls:
http://www.axolotl.org/
And of course you can always pop into the forum with any questions you may have - unlike our axies we don't bite!
Cheers Jacq.