OK, DOES and DON'TS

IF you are an experienced EE then that simple rule does not apply - but then you would not be asking the question.
If you have little knowledge of SMPS design, then it is a good rule to follow, and will always work for you.
John's comments are very much to the point.

If you already have an SMPS unit you may be able to make it work in a particular situation by placing a lot of inductive filtering on the downstream side. This isolates the PS from the driver as far as RFI goes. Things like an HP printer PS or a PC PS have to meet FCC requirements, and they do this by adding the filtering - before the output leaves the metal case. The RFI stays inside. This does raise the cost of course. However, cheap generic SMPS units from eBay are unlikely to have this sort of filtering. Caveat emptor.

I will add that if you want to be able to hot-swap part of a dual PS, it is fairly easy to do IF they are both simple transf/bridge/cap units and the transformers have the same outputs. - but I would recommend diode-isolating each unit.

Example: I use a MOSFET SMPS to drive my spindle, but when I first installed it (replacing the dead previous supply) everything else stopped working reliably, and a nearby FM radio went bezerk. Massive RFI. I added a large dual-winding (ie balanced) output choke right at the output terminals, and harmony was restored.

Cheers
Roger