Im not sure i follow all of this post but one thing that stood out is that you will need machine frame members much larger than 20 mm T Slots! A lot larger actually, it might also be cheaper to go with steel. You will want to read some of the build logs and define what your expectations are with respect to the machining. In the end your expectations define how you build the machine.
In a nut shell machining is nothing like a Reprap as you have to deal with reaction forces from the cutting tools. That and stiffness or the resistance to vibration dramatically impacts surface finish.
As far as ACME screws go do realize that you also have ball screws as an option. Which is better is open to debate but in either case you will need protection from weld splatter. The same goes for your linear rails.
Third if you dont know what you are doing industrial lasers are extremely dangerous. I would not even bother with a laser facility unless you are willing to fully invest in learning all the issues involved, the standards you must build to and the legal responsibilities in your area. You can end up blind or with severly damaged eyesight in a blink of an eye and maybe more horrifically an innocent bystander.
All of that being said the future is in fiber lasers. I really doubt 400 eatts input power is enough to do much with aluminum. Further it doesnt make sense to me to try to turn a router into a welder. At least not as i understand your post. Now cutting might be a different story but there are dramatically different needs with respect to the table (work holding) and the Z axis tooling.
If at all possble avoid mirrors! They become a maintenance nightmare
Designing interchangable End Of Arm Tooling ( EOAT) is pretty common in industry but the change must make sense with respect to the rest of the machine. You can get around some complexity here by having room on Z place two tools.