Hi Curt - With the linear rails mounted on a bench "side" you have to be careful as you have no way to get them parallel unless you machine the rail lands. The workspace vs footprint issue is always a tug of war. All you can do is either define your workspace requirement and end up with the footprint you get or define the footprint and get the work area you get. Then adjust. It will take at least 10 or 50 design cycles if you have not designed a CNC before to get to where you want to be. With linear cars they need to be at least two lengths apart prefer 3x.
There's lots of commentary on cutting aluminium on this site. I suggest you look at a high rail design (your 2nd image is going that way) vs the column design ie don't have columns, set your rails at the height so the gantry goes straight across, no columns.. This does not affect your footprint or your work area but does improve the stiffness of the machine dramatically. If its a benchtop machine and you have the space look at a fixed gantry design, this maximises the stiffness of the machine but has a larger footprint vs a moving gantry design. If this is a construction aluminium section based design I suggest you buy one off the shelf. It will be cheaper than developing it yourself then you can do better when you design/build your second machine. Even make your own parts etc If you are going to invest a considerable amount of time designing your own CNC do not use construction aluminium sections. They are too much of a compromise vs your effort and the result will be close to someones kit. Plus you need to put effort into the tool holder area before you commit to the base area. If you start at the base you will run out of room for the Z stuff. You have to look at the whole picture often.
If you want a small router to do work buy one, if you want to D&B do that but don't use construction extrusions. Having been thru the exercise of designing 20 machines plus and committing to build 3 of these designs they all have cost more than expected. Then looking at commercial kits they are about the same price as I build a one off for. But my machines have always been better then the same cost kit. That's mainly because I use good quality linear rails vs extrusions and wheels. You will find these things out eventually, sourcing parts at the right price will be the main issue with a build like this. So let the people who have done this before do it for you if you just want a machine. I have recently been buying rails/cars from a company and they ceased stocking a certain chinese line so the price doubled for their korean brand. Really threw out the budget!! Then I bought integrated stepper leadscrews from a company for a production model and they told me replacement antibacklash nuts were available. I built three machines and decided to stock spares so ordered nuts... Sorry we don't stock these!? Try getting 2 start Tr8x4 nuts somewhere!! So welcome to the CNC club!!
Write down your machine objectives and budget and specs and review often. You will drift... Ideally Do not build or buy anything until you have a complete design resolved on paper and costed so you can make the buy/build decision clearly. If your a Maker and that's what you enjoy throw these guidelines out and keep Making...Cheers Peter