I have a new Excella too and lucky for you I have gone thru the learning curve since I never had a Miele DW before. In fact I just had a service man here to respond to some issues and I pumped him for alot of information that the use and care booklet doesn't address.

To answer your questions:
Salt - Run your dishwasher w/o salt; there is a button on the settings menu which will tell you what your water hardness is. If it reads 4 or under several times you don't need salt.
Jet Dry - absolutely fine
Cascade Gel - Fine too, however, based upon what people have written here and the service man's advice powders may produce a cleaner result. I have been using the Cascade Gel myself.

Miele or other specialty brand products are not at all necessary; except the salt.
Good Luck
PQ

I have the G2830 LaPerla similar to the Excella.

Detergents:

I alternate and use a different one each week.

I use powder - Electrasol with chlorine bleach(non enzyme), or Cascade Complete(enzyme).
I had a disaster with Seventh Generation powder which left grit all over everything including the insides of the dw.

I use Liquid Gels - Cascade Complete Gel.
I don't use Electrasol Gel because it left a film all over everything.

I use tabs - Miele tabs (1/2 to full tab depending on soil level), or Electrasol Red ball tabs, or Electrasol tabs with Jet dry, or Cascade Action Pacs or Electrasol Gel pacs. No problem with any of these.

When using Gels I set the detergent selector to Liquid.
When using Tabs I keep the detergent selector set to powder. Most powders, tabs, gels and pacs contain enzymes. Electrasol tabs and Gel pacs contain enzymes. Electrasol powder and Gel in the bottle do not.

Finish/Electrasol has released their Quantum tab to the USA, although I haven't been able to find it in stores where I live.

Rinse Aid:

I use Jet dry Green apple with vinegar, I use Jet dry turbo, I use Miele Rinse Aid. I alternate when filling the dispenser. All of these work the same.

Salt:

I use Somat salt or Miele salt. Both work the same.

My Dw measures the water hardness when filling as does the Excella. Like PQ said you can find your hardness in the settings menu. If the DW needs salt I think it will ask for it and if the water is consistently below 4 it won't ask. The softener adjusts the the hardness automatically and only softens the water to the extent it needs to during each fill. Between May and October my water hardness was averaging 19 gpg and the Dw performed the softener step(periodically performed before the first prewash, to regenerate the softener) every other week while asking for salt at least once.

Now the hardness is down to 2gpg and there has been not been any softener step performed nor has it asked for salt.