I feed earth worms during the summer months - I don't culture them myself, rather I will buy 2 dozen at a time from a fishing bait store, then keep them in a tub of nutrient rich soil in my fridge for a month. Rinse and repeat! I have three axolotls and 2 dozen night crawlers lasts them about 4 weeks.
I've also had great success feeding small guppies, less than an inch long. My salamanders go wild for them and absolutely love chasing/eating them. None of my Lotls have experienced any digestive trouble with guppies. I tried Pet Smart feeder gold fish - one of my Lotls had an upset and wouldn't eat for a week, so I quit that pretty quick. Then I also tried fish bait chums, which are live minnow fish. Again, he seemed to become ill for a short time so I stopped giving those.
Small guppies seem to be the safest
It's also beneficial for them to hunt/chase live prey, as it mimics their natural behavior in the wild. But my oldest salamander is about a foot long - he's quite large and robust. The other two are maybe 6" and they manage to devour small guppies without problems.
Additionally, I feed frozen brine shrimp cubes all winter, as night crawlers aren't readily available. My Lotls are fat, bushy gilled, bright, and active - they get about a cube each, every other day. Gets the water a bit dirty more quickly, but it works well otherwise.
I've had a lot of trouble obtaining salmon pellets, and apparently it takes a bit of time to teach Lotls to eat them. I have always hand-fed my salamanders so they respond to me at the tank, expecting their feeding tongs!
I have even fed real fish organs, from the fish I catch in lakes. Worms are regarded as the most nutrient dense because of their organ meat, so I tried the real thing. They weren't too keen on it, but I had them eating wild fish livers, hearts, etc.
I've also read that you can feed axolotl larvae chicken egg yolk that has been hard boiled. I tried this today with my new larvae and they didn't seem interested, but other people have had great success with it.
I think some people exaggerate how "sensitive" Axolotls are. In my experience they are very hardy, robust creatures and bounce back very easily. At least when they're properly cared for otherwise!