What a nice idea!
Here are my thoughts, in no particular order:
1. For all the nuts and bolts, I'd go check out the big student exchange programs and see what they do, then pare it down a bit. But not too much. Most of those questions are there for a really good reason and they've had years of experience.
2. Extend your stay to three weeks. They get over jet lag after a few days, then start having fun, making friends and then its time to go back home.
3. Make sure that the clubs mirror what the students see at home. For example, a really serious fencer might hate going to a rec club once a week.
Which leads me to the question-
4. What is the goal? Exchange of students to learn different fencing techniques/experience what it is like to train in another country OR is it purely cultural with fencing as the common denominator? Or both?
5. Make sure your fencers/families have
similar habits. Does the student/family allow smoking? drinking? unlimited use of the internet? (I'm going to show how prudish I am and not include S-E-X, b/c of course, no one could hook up and get that far in a relationship in three weeks...right

?) Does the family get up at 5 and go work out? Only go to the club for 1 practice a week?
6. Pets/allergies/food restrictions?
7. You should expect families to provide the fencers with their own bed (not the sofa). Therefore, girls stay with girls, boys stay with boys UNLESS they are guaranteed a separate room or room with a sibling of the same sex.
8. Several "send you home no questions asked rules": A. no driving of any motorized vehicle, B. Underage drinking, etc. (I know that last one kind of clashes with #5, but when we hosted a student, those were the kill buttons.)
9. Make sure the fencer's insurance will be valid in the country in which they visit.
10. As coordinator, you will need to be able to be reached 24/7 while your students are traveling and in country. And have a back-up.
11. Be very clear what the fencer will be responsible for paying for-club dues, local tournament fees, their own way to the movies, etc. Or will the family include him/her as a 'family member'? Also be very clear that the fencer will be expected to help with regular chores but NOT become the dishwasher for the entire time-unless it is a regular chore and is shared with the rest of the host family.
12. I think background checks are in order for all host families-
13-EDIT-Language skills! I almost forgot! You'll have to include what level French/English everyone speaks! Might be a good thing to think about.
Good luck with your program! We would be interested in hosting a student!