I'm sure it will come to no surprise to anyone when I say that I am a huge fan of multi species enclosure, IF they are done well and properly. Sadly, most are not. I have done a lot of research on this topic and spoke with a lot of very experienced people about it. There are several keys to doing it properly, with space being the number one thing that most people bring up. But I have seen small 29g multi species enclosures that were successful. I have also seen large enclosures that were not. A lot of it goes towards species selection and research. But much more important than anything else, and very few people talk about this, is experience. I will give an example:
About a year ago, I had planned on building an enclosure for mantellas and a small uroplatus gecko species. I had no doubt in my mind that I could build a very dynamic enclosure since I was working with a 125 gallon tank. I had broken sight lines designed in, water supply, and even a basking spot for the gecko. This was all fine and dandy, and then I realized, while talking with my friend who breeds uroplatus, that I had zero experience with this species. In fact, I had limited experience with the mantellas. How was I going to know something was wrong in the tank, if there was? I wouldn't. But a very experienced keeper would, and that was not me. So the plan got scrapped. I may build it a decade down the road, but lost likely, not.
My point being that only the most experienced in whatever animals are chosen to go into a multi species tank should even attempt it.
You asked about shrimp and minnows in a water feature, they should not be an issue. I have kept shrimp and fish in water feature with my frogs with no problems whatsoever. Just remember to take I to account the size of the fully grown fish and the amount of USABLE aquatic area. Just because you pour 8 gallons of water into a tank with a water feature does not mean you are working with that much usable area for the aquatic critters.
Just my $.02. Hope it helps.