Different orchids do need different mediums. I use different sized coconut based mediums for all of my plants.

Most plants will do fine in most mediums. However, some people like to have all their plants on the same watering schedule, and for that you need different mediums. I know this is not exactly an answer to your question, but it depends on what your goals are. Keeping culture simple with one medium means that you have to learn each plants schedule. Light, pot size, what the pot is made from, air temperature, humidity all cause watering to change.

Your basic question, is there a particular medium for each specific orchid, the answer is no. Even keeping them in the original medium may not be the right thing, since a commercial greenhouse with high light and optimal temperatures can use a medium that would be sure root rot at home.

Actually, it isn't as simple as a one size fits all orchid medium. There are primarily two types of growing environments for orchids. Those that grow on the ground (terrestrial-semiterrestrial) and those that don't. (epiphytes). I agree that you can use one type of medium for each and just gage your watering but you won't have much luck trying to grow a terrestrial orchid in a mix for epiphytes and vise versa.
In other words don't try to use the same planting mediums for your masd as you would for your catts. No amount of water monitoring will make either happy in a mix designed the the other. So depending on what you're growing you may be able to stick to one medium but if you aren't sure what type of orchids you have or are all over the map with what you are growing than you might want to stick to your current method of transplanting into a like medium.

Another way of looking at it is that all of my orchids get exactly the same watering. If I want a plant to have less water I use larger pieces of whatever. If a plants wants more water I put it in moss or smaller sized medium.

Not the correct way of doing things but it works.

Nick

In addition to the advice above, I think you need to do more reading on the different types of mixes and the advantages to using then. I would say the most popular potting media these days are 1) bark 2) coconut husk/fiber based mixes 3)sphagnum moss and 4) pro-mix types mixes. Each can be amended with additives such as perlite, charcoal etc, usually to improve aeration and drainage. The mix you use depends entirely on your growing environment: 1) warm/cool? 2) indoors-windowsill/lights or greenhouse? 3) types of orchid grown-need to dry out or remain moist?

Also factor in the type of waterer you are as how you water you orchids is intimately tied to the type of mix you use. For example if if you tend to overwater, go for a more free draining non-moisture retentive mix. If you can cnotrl watering, sphagnum moss can be great.

Best to start off with a similar mix for all your orchids until you get the hang if it. As Nick say, amend the mix depending on a) the plants needs for moisture and b) the size of the pot.

Here is a link that might be useful: tips on watering your orchids