@Ashez, what I meant is that you could have a regular 1KC (only goals scored while under your management count) and a variation of that. Lets name it 1KC Variant (only players whose nation isn't available for management in game).
Whoever wants to do a 1KC only has to state in the career if it's a 1KC or 1KC Variant.
This way, for me, is the fairest because it allows you to pick, let's say, a chinese striker and do a 1KC with him and allowing the ITN goals to count.
Anyway, my advice for anyone wanting to do a challenge, play it the way you want. In the end you'll either fit the rules and enter the leaderboard or you won't. But the most important thing is for you to have fun while playing it.