With all the plot holes in science fiction these days, not just Star Trek, having a slave race construct a ship like the Scimitar isn't that much of a stretch when you think about it (OK maybe it is but work with me here). But for the sake of argument, lets just assume somewhere in the future the Remans are a major force, or at least have obtained the resources and prestige to build a small fleet of ships for their own uses. There, that explains where Atolm's ship would come from, though who needs an excuse to design/build a ship in the first place, aside from just wanting to?
There isn't a set rule for what makes a Trek ship a Trek ship other than the fact it most likely has warp engines, a bridge, and phasers/disruptors/pea shooters of some kind. Having ships that stray away from the same old spit just makes sense as not every race in the galaxy has the same design tastes. The less humans like and relate to a ship, the more alien it is. All those glowing balls of light and flying pinwheels from TOS are good examples (budget constraints and low-tech special fx aside). Then again, I'm a sucker for jet-black ships that resemble something the Grim Reaper would fly so I'm already biased.
Besides, Halloween is next week and ya gotta scare those kiddy starship captains somehow