Hey there Medickep, I like your great, free bench and use for the RAS station. Before you get too much done on the set up, I highly recommend that you go visit the DeWalt RAS Forum on Delphi Forums
. This forum is a great radial arm saw resource to say the least, and the very first thing you should do is to check out their FAQ's, which will guide you in producing a new and better top for your RAS (keep the original as a template). In short, they explain how to create your own super flat, super strong, and super stable RAS table top AND the right ways to tune it to be a wonderfully accurate, highly capable, and pleasurable wood machine. I routinely use my 2 RASes (I got rid of the giant cabinet saw) for ripping, dadoing, jointing (don't have the space for one of those), and moulding. The RAS can also be easily used for planing and as both a disc and spindle sander.
If you follow the forum's suggestions, many of your questions about sizing, positioning, and successful use will be answered; you'll also end up with a tool that can do everything the sliding compound miter saw can do, plus a lot more. After a good tuning, you'll be able to do far more with your RAS than your SCMS can pull off, and you may decide to just reserve the SCMS for mobile project situations and NOT include it your bench plans. That would be a great thing, because while a SCMS fence stays put, the RAS's fence is regularly shuffled around to accommodate things like ripping wide and narrow stock.
The key to success on the RAS is to build a proper table top for it, and the site explains the why's and how to's. A very important characteristic is that the saws are designed to have a series of different width back boards which are shuffled about so as to allow the fence to be placed at various distances away from the front edge of the RAS which helps the ergonomics and safety for performing various crosscut and ripping sizes.
1 CRITICAL concept: Zero-clearance kerfs, and that means the fence and the table top skin are both consumable and moveable. Having a crisp and precise kerf is a great tool for quickly showing exactly where the workpiece/cutline will be cut, and of course it yields tearout free cuts. So start by rebuilding your RAS table top as a reference grade foundation and then complete it with an easily replaceable skin. I prefer to use a non-attached skin (a thin piece of scrap) so that I can slip it this way or that, and flip it over to repeat on the back side, in order to always have a crisp kerf. For most of the guys on the RAS Forum, they tend to tack their skins down because it looks nicer, but then getting a nice fresh kerf is always more of a chore, uses more material, and that all will lead to laziness about the kerf and less precise and rattier workpieces.
If you follow these ideas, the idea of having both the RAS and miter saw in the same top will take some more thought. For instance, the MS wants a fixed fence and the RAS wants a moveable one-the MS and its fence will quickly get in the way of cutting long and wide material on the RAS. If you choose to use a fixed fence w/ the RAS, then you will be choosing to loose a huge part of the RAS' capabilities and it will be just a larger duplicate of your MS, an unfortunate compromise indeed. Incidentally, the MS benefits from a consumable fence too for the very same reasons, but making bevel cuts on either tool will quickly kill the standard crosscut attributes of the fence and that's one reason to change out that section of fence.
This is pointing to using separate fences for each tool, and that's fine because you have the space. You'll just need to figure out how to move one or both saws so that the MS doesn't conflict with the RAS's fence action in its rear-most position. Have fun with the new toys!