There might be 1 knowledgeable employee at a fish store out of 50 employees. Even at good LFSs the percentage of employees that i meet that know what they are talking about are low. The difference is that the Petsmart, petco, and non-fish specialized stores "train" their employees a couple of basics about fish, and those employees in turn apply those concepts to ALL fish, which is untrue. Moreover, they can only work off the facts sheet provided to them, ie. ive seen petstores with Bala Shark labeled at 4 inch max size so its only antural for the employee to think, well its only 4 inches and at 1inch per gallon u can fit 5 into a 20 gallon tank (20 gallons will house 0 adult bala sharks). Why does this happen? Because most bala sharks sold will die at 4 inches, not naturally, but from living in an unfit environment, ie. tank too small. However, most people will go on thinking it was natural cuz the store said 4 inches and it died at 4 inches so it must have been full-grown.
Also, its not really a "bad" lfs, even if they are specialized in fish-care, to give you bad information. Its hard to find employees who know a lot about fish care, however the same couple of people cant work everyday for all 10 hrs the store is opened so its always good to take a LFS's advice but remember to do your own research.
I think StampedFish is already a couple of steps up from many fish-keepers from having, at the very least, asked about the conditions of his tank, and willing to rescue unwanted fish. He/She just needs to be less stubborn and realize that the optimal conditions for his fish are not met in his tank. Attacking him/her wont change his/her mind.
As for this topic, I see you have a community tank stampedfish, you could easily risk slight overstocking and add a few more guppies. They will breed and you can take the extras to feed your other tank. Other options would be frozen foods, personally, i dont enjoy the sigh of living animals getting torn apart so i stick to my frozen foods :-/.
Brine shrimp require their own hatchery to keep up production. However, they are low in nutritional content so they should only be fed as treats.