I wouldn't worry about the cables with the bike that has "exposed brake/shifting lines on the underside of the triangular tube". The cables should really be fine. I'm not 100% sure what you mean by a "triangular tube", I'm not sure if you should be concerned about weird shape of the tube just for getting it to stay on the rack purposes. My Saris Bones rack (not a hitch rack) doesn't have a problem with my strangely shaped carbon top tube - differently shaped top tubes are so common nowadays I probably wouldn't worry about it as rack designers have gotten to used their rack needing to hold bikes with differently shaped top tubes.
Other than that, good luck!