most of that is just talk to scare you into using their company. If you haven't been to Cambodia before I suggest you take a bus and spend a week or so in Siem Reap.
Going with a company just means more money. It doesn't improve your chances of getting in. Why are you worried. You seem to have done this enough on your own to understand how it all works. Take a train from Bangkok to Poipet.
Some lines but they went fast. The people were very helpful. Nobody tried to con us for money just told us what line we needed to be in and what we needed