Hi Tom - I'm new here - but not new to turtles. I've been keeping turtles for several years and have raised a few spotteds from hatchlings. I've spent a lot of time thinking about doing a mixed tank with spotted turtles - from the outside they really do seem to be prime turtle candidates for such a set up. They like shallow water, don't tear up plants too much and so on. Here comes the but. Spotteds can be a bit quicker than you think as they get older and are more fragile than they seem when they are younger. I've kept spotted turtles with fish and snails - the fish are fancy guppies and the snails have either been the so called mystery snails or the little hitchhiker types that look like large dark grains of rice. Anything that moves is potential food and mine eat just about everything and anything that moves - and that is why I haven't added frogs - didn't seem like a healthy prospect for the turtle or the frog. During the summer outside I've seen some spotteds consume tads as well, surely they would try to tear up a frog if the chance presented itself.
While I find spotteds to be one of the most beautiful turtles some of the smaller bottom walkers would better suit such a viv as you describe, stripe neck musks come to mind - smaller than spotteds, better swimmers and they almost never come out on land (nesting only IME) choosing instead to rarely bask partially submerged and generally shyer by far than spotteds thus less likely to chase down a frog. I've seen similar sized spotteds and musks react differently to prey such as earthworms or fish -spotteds work themselves into a frenzy chasing them down and the stripeneck musks usually run and hide from anything but snails.