You could have the heaters and HOB skimmer in the DT, that's not a problem. The main point of a sump is to add water volume but one of the luxuries of having a sump is to keep equipment out of sight. Also, I would still suggest accommodating enough room for back flow: you will have to power down your return pump at some point to do maintenance and what happens if your pump fails some day when you are at work? What are the dimensions of the DT? If you're just in a hurry to get the actual sump tank then you can get the tank and we can help talk you through setting it up properly. It's t a little confusing at first, but once you start to understand it it gets easier.