Hi Sebastion - Now I have seen your images I'll answer your Q's:
1) In a word no. It will make the machine very costly for the purpose (which you need to explain better then "I want 500mm z") it has redundant rails and cars and a complex gantry to keep square and level. Due to ballscrews being slightly different they do wander over time due to this in-tolerance. This will take your gantry out of level over time.
2) again in a word , Not worth a try. Since square rails oppose moments, having extra rails does not help. The first rail in the loadpath does most of the work. Very large machines (like size of houses machines) have double rails but this is so they can be assembled with stability not to improve or decrease the loadings
3) A machine can always be made "strong" enough to do the job. The actual forces on cnc machines such as this are quite small. Even small carriages can support tonnes of weight... Machine rigidity & simplicity are the things to chase and that will give the machine its required strength.
So its your first machine, good on you for trying
. Firstly a good machine is made to a fulfil a purpose. If you try to design a general purpose machine the compromises are too big and you go round in circles and eventually end up with a machine that's crap (I've been there) or a machine that looks like the usual machines. The usual machines have been refined over a long time say the last 100 years to get where they are. They are deceptively simple, so simple, you say I can do better eg use twin rails for a design vs a single rail. So A) define what you want the machine to do as this will answer many of your Q's & give you a direction that will arise often in the design journey B) find a commercial muse machine that does what you want and contemplate why it is what it is. It will inform you of many things that will reduce your thinking stress and project time span...
Regarding a mixed media build. I recommend you stay in one material universe. Mixing materials may get you into trouble with thermal expansion differences. eg steel expands 13um per degree vs aluminium at 27um and concrete about 13um...These sound small but they do make a difference in a machine and commercial machine builders go to long lengths for thermal stability and so should you. I'd forget about filling steel tubes with stuff, commercial machines do not do this and this forum has 1000's of discussions about this topic. My view is use a thicker tube, its then stiffer and easier to do things to in future and far cheaper... and banging parts with hammers and listening to the ring or soft ring does not really answer the question about the machines dynamic stiffness, its far more complex than that... Peter