Well this is not going to be the "dont do it is bad" post. It is some background, because things need explanations, not just statements for people to understand. Here it goes:
If your room temperature stays in the 70's-high 60's and your area has good climate control tecnically, temperature wise, it is tolerable, but not advisable in the long run. If you care about welfare of living creature more than "lookin at something pretty" the mixing of these two species, based on geographic origins and temperature needs makes the scenario one that should be avoided. Just because you can put something together does not necesarily mean you should. If you have many years in an aquarium setting, and have biology degrees, animal behavior, evolutionary sciences, worked in a lab of scientific instituions, etc, you may know something I do not. If you do not have all those credentials leave this sort of experamenting to people who know what they are doing.
Platies are from a tropical area and it is suggested ideal temperature is 18 – 25°C. At least with wild platies. If they are sword tails, they are hybrids for the pet industry, but i do not know much about X. helleri specie of the hybrid. It is even stated that platies live in warm springs.
The domain of X. maculatus extends from Ciudad Viracruz-Belize. This is more southerm and more costalthan the realm of axolotl. Totally differnt habitats.
Axolotl are native to the water area of Cuidad de Mexico. This area is the valley of Mexico which sits on a high plateau. Just because Xochimilco is in Mexico doesnt mean the water is warm and a "tropical" in temperature in the sence.
"The water temperature in Xochimilco rarely rises above 20°C (68°F), though it may fall to 6 or 7°C (43°F) in the winter, and perhaps lower."
In other words, axolotls do not tend to live at similar temperatures as platies. Infact, there is barely an overlap. Just because you state that axolotls are exothermic does not mean it is ok to keep them in the high 70's range for too long (infact dont even keep them above 20°C if it is possible. There are heat related illness due to prolong periods in high temperature. It causes stress also. A stressed out animal tends to succumb to injury, illness and eventually death. heat+stress=immuosupression and axolotls have an interesting immune sytem, but if you keep them in "bad" condition, all their wonderful regnerative and healing properties they evolved with wont help too much.
If you want the platies to "feed" the axolotls, raise them in a seperate tank. Usually "food" is grown in seperate areas and only introduced in feeding. If you are really set on keeping other creatures with axolotl find one with a closer temperature range. Plants are nice too, if you want "something pretty to look at". Please read the articles on Caudata Culture and reputable amphibian sites, especially articles relating "mixing species" and "diseaes" I am sure you can find something relating to axololts and shrimp. I know on caudata culture the articles about keeping salamanders with other creatures is mentioned. It says which animals are ok and which are not. If you want a nice looking display tanks with axolotls, i sugges some plants, nice rock works, nice scape first and foremost. If you think it is still missing something, add some other critters that are recomended in a FAQ's section. I suggest you give it a read, incase you do not want to get into details listed here is what it has to say about fish
http://www.caudata.org/cc/faq/FAQhou.shtml
"What type of fish can live with my newt?
Most fish in pet shops can NOT live with newts because they need much warmer temperatures than newts do. Tropical fish can’t take the cold temperature that is so healthy for most species of newt. Listed below are a few fish and other animals that would be OK to add to your tank. However, watch for any signs of stress to the newts, and note that a few newt species are able to catch and eat fish, snails, or shrimp.
* White Cloud Minnows
* Guppies**
* Zebra, Gold, and Blue Danios**
* Scissor-tail Rasbora**
* Ram’s Horn Snails
* Freshwater shrimp
* **These fish species are suitable only if you have a species of newt that will do well above 68°F (20°C) year-round. This is pushing the lower limit of tolerance for these fish, and pushing the upper limit of wintertime temperature for most newts."
here are the "mixing disater stories" several fish species are mentioned with nasty results. I guess you can use that to putting fish on your list of "what not to keep with my salamander"
http://www.caudata.org/cc/articles/Mixing_disasters.shtml
I hope this helps with some of your questions and able to understans answers to other.