I agree with others that switchers are a better choice in terms of efficiency, but they can be somewhat complicated to deal with if you're inexperienced, and there can be lots of weird effects that aren't immediately obvious (precharge sinking, beat frequencies, etc.) that can make life difficult. Assuming you've figured out your power dissipation and know how much current each rail can deliver, if the linears will work for you, stick with them (at least for the first pass).
If you're trying to achieve a variable-amplitude square wave output on your adjustable rail, the chopping may introduce noise into the main 24V rail, which could show up on the other rails. You may want to have an LC filter between the main 24V rail and the regulator input to provide high-frequency isolation, and will probably need extra capacitance on the adjustable regulator output (bulk electrolytic as well as low-impedance ceramic) if you expect the square wave edges to be sharp.
1, 5) There are some dangers with your scheme.
Power dissipation in the linear regulators will be
\$(V_{out} - V_{in}) \cdot I_{out} \$
which is significant, especially for the lower output rails. 78xx-type regulators have built-in thermal protection around 125°C, and (without heatsinking) a junction-to-air thermal resistance of 65°C/W. Your thermal management will be challenging.
Another potential problem - if the series-pass element in any of your low-voltage regulators fails or gets bypassed (shorted), you'll present the full 24V input to the output. This could be catastrophic to low-voltage logic. You should protect your low-voltage rails with SCR crowbars that can sink enough current to put the DC/DC brick into current limit and collapse the 24V rail (they'll need big heatsinks too). Fuses are unlikely to be good protection since the 24V brick likely isn't stiff enough to generate the \$I^2 \cdot t\$ needed to blow a fuse.
2) Whatever floats your boat.
4) Meters aren't huge loads. Just use one of your rails.
3) Correct - all regulators have headroom requirements. If you want the maximum 24V out, you'll need a direct connection, and will have to rely on whatever intrinsic protections the brick will provide you.
If the switching regulators have common ground between the input and output, you cannot put them in series.
If the input and output grounds are isolated, you can put them series.
What is limiting your 42V input to a single regulator?
If it is the input capacitor, replace it.
If the switching device, MOSFET mostly, cannot handle 42V across its source and drain, replace it with a one with higher rating.
If there is no MOSFET but an IC doing all the stuff, connect a MOSFET (with proper rating) to the switching output of the IC and move the inductor and capacitor (filter) to the output of the MOSFET. Make sure that the pulses' peaks coming from the IC are within the specification of the MOSFET. Too low peaks will not turn on the MOSFET, too high pulses will damage the gate of the MOSFET.
Best Answer
I have used 2 7815 regs to make a plus minus 15 volt rail for some Opamps .So you could do the same with your 7805 regs to make the plus minus 5V you want .I used a 50HZ mains transformer with separate windings which are common .An advantage of doing this rather than the orthodox CT winding and 7805 7905 pair is better stability on the negative rail .I have found more stability issues from the 79 negative series that the 78 positive series.Better stability gives better predictability.The 79 regulater is featured in the www.badbeetles.com website.