There are always a lot of Kuroshitsuji cosplayers in France. I think that you can meet Kuroshitsuji fans during Japan Expo!
About advices, what kind of advices do you need?
Food and water are very expensive inside, you had better take your own sandwiches and water bottle.
If you have to take RER, you should buy all tickets you need in advance (after Japan Expo, there are many many people waiting to buy RER tickets, it's better to buy return ticket in the morning. RER tickets have no hour nor day)
If you need cash, there are ATM. During the morning and the evening, there are often crowded, however during the afternoon it's ok.
Saturday is really crowded, you have to wait all the time. Sunday is less crowded.
Friday is calm. Thursday is calm too but there are less thing to do.
You have other advices on their website : Some advice for a stressless festival!