Using S&H with noise is common because it gives you a nice stepped-random, but you can use it on anything you like.

Here's a few ideas:
* Use S&H to sample a fast sequencer output slowly to produce a related slower sequence
* Use S&H to sample a LFO to get a stepped effect instead of smoothly sweeping whatever the LFO controls
* Use S&H on an audio signal triggered at audio rates (if the module can do so, not all can) for a "bit crush like" effect, except it's a crush in time resolution rather than amplitude resolution

The +12V and -12V rails are separate power busses, they dont' add together - you should be okay with those numbers as stated, though the common advice is to leave some headroom between your usage numbers and the PSU's capacity numbers, so you don't want to push it all the way to 3A on either.

You can't directly daisy-chain modules, but you can either make custom multi-drop power cables yourself or buy some "flying bus" cables which plug into one socket and have more sockets on them for individual cables. In my rack I made a few custom multi-cables just to simplify the cable mess inside! You don't want to chain power-heavy modules this way though, only the "small" ones, my biggest grouped cable is 4 modules totaling ~600ma on the +12v.

Does anyone know if the 'mix' output on the Rample is mono or stereo?
-- rextable

With only a single jack, it's almost certainly mono. Basically nothing is TRS/Stereo except occasionally output modules to headphones/speakers, when stereo does exist it's mostly as pairs of mono jacks.

I'm in a home basement in the US so I assume 15 amps per circuit in general.

You don't really need to worry about the AC circuit's current - it's 120V coming out of the wall, so 1A there would make 10A at 12V (ish, minus a bit for conversion losses) when it comes out the other side of your power supply. You'll have to build a massive rack for it to need more than a regular household 10-15A AC circuit to plug in to! :)