When I hear "youtube" and "8ft turtle tank" I think Bob's. If it was his tank that inspired you, you'll be hard-pressed to keep a plant as nice as his with sliders. they're much harder on live plants than other species.
But anyway... you can keep bottom-dwelling species like cory's and madtoms if you give them places to hide. Also, faster fish like danios and many other tropical species are fine with most turtles, as they tend to be able to turn on a dime and evade them rather easily.
As for another turtle, really any species will do in theory. Aggression in species isn't really that much a problem in sliders or other basking species. You might do well with another slider (scripta, elegans, or troostii) or a Cooter of some sort (redbellies, rivers, etc.), or even painteds (they're even more herbivorous than cooters and sliders... good for your fish!). Musks and muds aren't usually aggressive towards other species at all, and tend to occupy a different area of the tank (bottom, versus the surface area where the basking species tends to be more often). Of course, you'll have to make concessions for these guys as they aren't as happy about swimming large vertical distances as the basking species are.
As for gender, I'd go with another female if you go with a basking species. You'll be fine on space, and you won't have mating aggression between the males. If you go with a bottom dweller like a mud or musk, gender isn't going to be much of an issue.
Most other turtles will do fine, as long as their requirements are similar.
Soft-shells and snappers are probably not what you're looking for. Too big, carnivorous, and dangerous to be kept closely with other species, in my opinion.
Josh H
