If you stock one type of the fish, then you'll be able to catch only that fish, more you stock odds are dividing according to the level needed, like in all fishing spots. This means if you stock your pond with minnow and lobster, without bait you'll catch lots of minnow and only few lobsters. It doesn't matter if you stock lobster 5 times and minnow only once.
Quality of fish used to stock pond doesn't matter. You can use normal quality fish and catch perfect one, but using high quality bait, having maxed skill and skill challenges done makes a huge difference.
Also, it seems that non got "great spot" stocking pond in lot where Sim live, no matter how many different species you stock, or how many times you stock same type of fish. I manage to stock same spot in backyard with at least 10 different fish, and they are all listed when I inspect water, but still none is jumping up and it is still "descent place to fish".