If memory serves, Pokémon you import aren’t available to you until you reach Fuchsia City and the Park therein where you’d have to “catch” your imports. Granted what I’m saying is based off Pokémon GO and not the peripheral(Id assume it’s the same means but eh what do I know). As for being cheating, I wouldn’t quantify it as such since a level 50 Pokémon can be trounced by something that is 30 levels lower. Why you ask? Candy buffing the living daylights out of its stats.
Seeing as you can get a buffed out of its mind Pokemon by then, I hardly see the issue with bringing in imports. Heck they(TPC) overly encourage it in the hopes it’d boost interest in Pokémon GO. Plus you can play however you want.
As stated above, GO importing isn't available until Fushia City as it requires a special attraction called the GO Park but despite the somewhat late assessability I am pretty sure this relationship is the point of the LGs, that these games are largely just promotions for GO.
Be aware that, according to what I've gathered, the imports are permanent, once a Pokemon has been removed from GO it CAN NOT be returned.
Even game exploits in ANY game, not just LG, such as MissingNo and the Mew glitch in the original RBY that can be/are considered cheating aren't illegal. If that's how you want to play you're free to go ahead.