No need to limit an Aquam realia to water. The Form covers all liquids, so acids, poisons, wines, juices, and oils will all work as well. Of course, you'd need to replenish the juices and wines as they spoiled and/or mysteriously disappeared. 
burp 
For Auram I could see a collection of various types of smoke and other familiar gases in sealed containers.
Chris
Auram could also be any place that is exceptionally windy, or prone to extremes of weather - such as a mountain pass or narrow canyon, something like that. Wind, rain, lightning - that's all aurum too, and even if a bit hard to predict, so long as a spectrum of examples are available it should work.
Rego is a bit harder. One approach would be a large mechanical apparatus that could put "stress" on a central object, perhaps with a series of hooks, chains and pulleys. By pulling one way or another, an object attached inside can be bent/held as needed, and observed while this is happening. (A bit horrific if a living creature were placed inside, but all in the name of Magic and all that.)
But Rego can also encompass natural changes, so something like a kiln or oven that changes things put within it -from sand to glass, clay to ceramic, that type of change. I'm fFailing think of a full spectrum of effects possible without magic, but it's a start.
I was reading the rules on Realia, and there's written that when you divide a collection "Each smaller collection has a quality based on its number of items, and studying from it counts toward the maximum number of times a magus may study from the complete collection." (p.102, last paragraph on Realia).
So it seems to me that RAW, it is always a bad decision to split a realia. And it seems a good idea to wait for the realia to have reached its "final" size to study it (because studying a size 5 realia should count towards the number of time you can study it once you have increased it to size 6, as an example).
For a Rego example, maybe you could go with perfect crystals, or other "ordered" natural things (spiderwebs ?). You could also probably go with any artpiece, or shedding from diverse animals (snakes, insect, ...), as it is a natural change ...
No.
Yes... Actually no. {see below, realia B are the added bits that increases it to 6}
If I have a realia 5 and you have a realia 5, they are both independent. I can study mine to 55 then borrow yours and study it to 55. If someone borrows both and studies them as a single realia 6, each season counts against both realia 5. Only when an item is shared between realia are you right about it being a bad idea. Which, depending on how you interpret it, may for instance block sharing an item between a Perdo and a Corpus realia.
In fact, you could do a pyramid of realia:
- 6 times realia (6)
- 5 times realia A (5) + 5 times realia B (5) + 1 time realia A+B (6)
- 4 times realia Ax (4) + 4 times realia Ay (4) + 4 times realia Bx (4) + 4 times realia By (4) + 1 time realia A (5) + 1 time realia B (5) + 1 time realia A+B (6)
- etc.
Yeah, RAW yields to combinatory madness sometimes. 