Sometimes yes, and sometimes not. All depends on time of day, volatility in the market, type of setup, etc.
Slippage would be at most .50. You can always use a limit order 1 tick outside the bid/ask.
In any event, watch the "tape." You see 100,200,500 size executions everyday.
Consider that the ES trades ~ 2million contracts a day. That's ~5000 contracts a minute average. More in the mornings and afternoon.
There's other tricks you can play. But see how you do trading 50 first and as you increase size, you'll get a feel for when and when you won't be able to do that size using a mrkt or limit order.