I understand that aggressive orders will be sent to a market maker if I'm with TD Ameritrade or TradeStation and use 0 commission. What about Stop Limit orders? Will they also be sent to a market maker, or routed directly to a venue?
Once the condition is triggered, they become regular orders. Don’t worry where your orders are sent, worry if you don’t get the results you expect.
Likely the market maker handles both the stop trigger and subsequent limit order. Retail orders are increasingly getting "outsourced" to subsidize your free trades.