Re: defending against a boxer
This actually is an interesting topic.
It's much deeper than, "Oh. Can I get a kote gaeshi off of a boxer's jab?"
I will add, do not forget that shomen uchi, yokomen uchi, and tsuki, amongst other atemi are all legitimate aikido techniques. Combine the skills you learned as uke and nage! You must recognize the nature of the engagement that you are entering and respond in an appropriate manner.
Remember your principles: relax, be calm, centered, grounded, and move as one piece.
Watch your ma ai, and seek ki musubi and awase.
Purpose matters, form does not. Remain open as to the form and strategy with which your desired outcome may be realized.
And then, like Michael Neal said above, get out there and train it.