What kind of bad microbes can evolve when compost tea gets old. How will it affect plants that are sprayed with it? Does it become systemic and then does it make the food toxic? I ask because I have used some old compost tea that smelled okay to make Garrett Juice and the plants are thriving.

First of all, if it smells ok then I don't think you have anything to worry about. Usually the issue is that it becomes less active, the longer it sits. I don't think toxicity is something you have to worry about. Plants are thriving, all is well.