What kind of shiners did you try?
I think that Rosy Reds, which are a colour morph of Fathead Minnows (Pimephales promelas) would fit the bill, although a bit smaller than you specified. They're by far the most visible and easily-observed small schooling fish that you will find for use in a pond that is viewed from above. They're also practically indestructible and breed like mice, although your pond is pretty full of mouths already and likely very few fry would survive.
I didn't realize you had a Hi-Fin in your pond. Interesting in hearing how it does long-term. Mine has been outside in my dug pond for a couple weeks now and shows himself just often enough to kick-start my hopes again each time I start to assume he's died...
