You can do some Genera grouping but the species don't all like the same micro climate. I group mine more by light level then wet/dry.

Then you have winter to think about. Some orchids that can summer together don't want the same winter conditions.

You can do a lot with plant mediums to get watering cycles in matched up.

You may be catching my drift for your last question. No simple answer.

there are only ~ 30,000 orchid species.

carefully select plants which are appropriate for your conditions and for your lifestyle.

--Stitz--

All of the above. Some dendrobiums want that winter rest, so don't water them starting november, some don't. As far as "Keep this wet, don't keep this wet", really the simple answer is water when it approaches dryness. Evenly moist is the key: 90% of plants don't want to hard dry during the summer. The big question then becomes "how long does it take each plant to dry down?" As the question implies, it varies from one plant to another. You'll eventually find a pattern emerging of what dries down when. During the winter, you can let most of them get fully dry before watering (with exceptions of course). Again, though, the degree to which your plants STAY dry will vary from one plant to another.

You can also tweek your medium to hold a bit more water for those plants that like to stay moist, while waiting for other plants to dry out.

As Stitz suggests, carefully select plants that are appropriate for your conditions. The simplest thing is to grow plants with the same needs, such as the cattleya alliance, which like to be watered and then allowed to dry out before being watered again. If you want to grow a variety, then you'll have to bite the bullet and set up a watering schedule.

I have catts, Phals, and milts, and water once a week. sometimes I might go beyond that a day or two, or even three, but it doesnt hurt them. They bloom and look healthy so I must be doing something right. As everyone says its a trial by error, depends on what you are growing, your light set up, etc. I grow mine in the basement under lights, with a fan blowing. Hope this helps you some, good luck. Tricia

While cat sitting I over watered my daughters orchids...now the leaves are droopy...did I kill them???