When you use BrokerTransmit, C2 does not actually use your live broker account value to determine Collective2 performance stats. We use only the results of the trades you place (and the marked-to-market results of open positions you create). So you can add cash, withdraw cash -- whatever -- and it will not affect your C2 Performance statistics (and of course none of your subscribers will be informed about your cash activity).
The important thing to remember is: Does my C2 model account have the margin and buying power necessary for the trade I'm entering in my brokerage account.
One final note: if you withdraw so much cash from your real-life brokerage account that you can't place the trades you want, then of course your C2 strategy won't be able to place any further trades (unless you turn off BrokerTransmit, and use some other method to run your strategy on C2).