Noo! Of course it's not cheating!
When you bought that used Fire Red, it came with those Pokemon, they're now yours!
Of course, if you want the challenge and fun for yourself to train your very own Lv 100 Pokemon, by all means, train!
But, of course it's okay to take them! As long as their stats and information is legit, they're legit. There's nothing wrong with taking them, it's their fault for leaving those Pokemon in that game if they had wanted them!