1. ES is a very liquid product.
2. Given the question asked, I doubt the poster will be doing much more than say a 5 lot if he's never traded the product before (nothing wrong with that of course).
3. You will not experience slippage doing 5 lots in a contract thats typically 500 up either side - well less than 1% of trades anyhow.
4. If you had traded the product you are giving advise about you would know this. Therefore, I can't tell if you are serious or just throwing out 'advise' for ego. i.e. trolling.
Depending on the broker, you may not need to pay for Ninja. There is a "direct" version available for free that you can use for live trading. With the direct version you cannot use Ninjas ATM strategies, so this means your entries and exits must be manually placed which more than likely is what you would do anyway. You can always upgrade to the full license if you feel the need to.
The fact is he WILL experience slippage, unless he always uses limit orders. And that is even if he trades 1 lots. It may be slippage of a tick or two on stop orders (roughly 10% of the time, from my experience actually trading the ES). It can also be a heck of a lot more, when reports or breaking news comes out.
The other slippage many people do not think about is in the bid/ask spread. Many simulators will wrongly show you getting buying at the bid, and selling at the ask. The reality is the opposite. You can add about 1.5 ticks per RT for that slippage. This also has to be applied to any backtests from software like Tradestation, Ninja, MultiCharts, etc, which always give overly optimistic results.
If you have any questions please send me a Private Message or use the futures.io "Ask Me Anything" thread