Here is some feedback/thoughts:
1. I think item #6 needs a little clarification. Let me use an example to illustrate.
Imagine a situation where you had five positions each of which were initially 20% positions (stocks A, B, C, D, and E). Stock A doubles and then you sell it which gives you a $40K cash position. Stocks B, C, and D don't change in price but E goes up by 50%. The portfolio is now worth $130K.
When you go to reinvest the $40K proceeds, what is the maximum amount you can put in any new position? Did you mean for that to be 20% of the new portfolio value ($26K) or is it truly limited to $20K. One interesting dynamic this will create is that proceeds of winning positions will often be reinvested into multiple positions.
2. Are you planning on providing periodic updates on the content of everyone's holdings? If someone is doing 5 trades per month it might be difficult to keep track of what one has after a few months without an update. I'd hate to see this be a lot of work for you so whatever can be done to make tracking easy is a good idea.
3. How will people express the amount they want to buy or sell? Will they express it in dollars or shares? This is a minor detail but perhaps something you want to think about as it might impact how you present information in any updates.