OK - in that case:
No, they are not herbivorous.
No, they can't be kept safely with small fish. (And your fish choices need very different conditions from one another, AND from the turtles.)
No, they will end up fighting and/or injuring themselves sooner or later.
No, don't put them outside. They will get eaten, or escape.
And of course, you need to have an appropriate sized tank/pond, appropriate lighting, an appropriate supply of varied foods, temp, huge amounts of filtration. they are demanding animals to keep, if you want to take care of them properly.