First of all one of the most important things you can do is work during the off-season to get your team organized. If you have 40 people show up to kickoff next January with relatively no idea what is going on you will most likely have chaos on your hands. I suggest having regular meetings over the summer to develop skills and to help kids find out what the like and don't like. Then have your team leadership work with some of the student leaders (not necessarily elected leaders but students who took initiative this year) to develop a team organizational plan. The Simbots have a great workshop on this found
here.
After the students learn about all of the different sub-teams let them sign up for sub-teams they would like to be apart of. I would let them sign up for as many as they are interested in and once the work starts they will settle into a couple they really enjoy. If you find you have an immense imbalance in your sub-teams, you can ask students if they would be willing to work on teams that need help. The most important thing is to make sure everyone is enjoying what they are working on. If they don't they won't be getting the most out of their FIRST experience and more likely than not your team will be getting less than optimal results.
As far as your specific team is concerned I know nothing about you mentor base but I would also suggest having a specific mentor that either leads or can help the sub-team in case they run into any issues. This will keep any one mentor from becoming overwhelmed. Along with mentors it may be helpful depending on the makeup of your team to have student leads in all of these groups. It helps individuals develop leadership skills and gives the sub-team someone to look to for direction. This student lead and mentor should work very closely in order to make sure each sub-team is accomplishing its goals and that those goals are in line with the overall team goals.
These are just general suggestions and if you have any questions feel free to PM me or just post back here.