How many team leaders can I assign to a squad-size element?

You can arrange them however you want but talk it over with your NCO first. I would stick with the original MTOE layout.
However, in order to allow your people leadership chances assign them tasks and make them the MFIC for that task. You can also make them give classes to each on the different systems and functions in your shop.

I am sure your motives are genuine and you simply want to get your subordinates OJT leadership training. While I agree with others that you can do almost anything you like, I recommend you stick to your units organizational structure. A sign the senior members to the NCO positions (of course with the concurance of your leadership). You then coach and mentor them. As for the your desire for all of them to lead, do not assign lase positions, there are a multitude of tasks to be completed in a daily basis in any Company. Simply a sign them, or volunteer them to oversee these tasks, providing guidance and support ensuring there success.

There are many ways to develop your Team.

If this is your squad and you want to have people supervise others, you can arrange that any way you like. If you want the Privates to supervise the SPCs you can do that. If you want to know the way your section is broken down in assigned leadership positions, then open up FMSWEB, look at the positions, the slot you are filling and the ones left to be filled