growing in coco prof

Not really sure about the coco/clay-balls ratio. Don't the setup instructions for the wilma system give any advice on this? You should be able to find them online, even if they didn't come with the system.

I haven't really seen a system that mixes the two mediums. Coco is usually fine by itself, as are clay-balls.

At a guess, you could try a 5cm layer of clay-balls at the bottom of the pot, to aid drainage, and fill the rest with coco.
Using google, I've found growers advising a 50/50 mix of coco and clay-balls.

Googling '8 pot wilma system instructions' leads to a PDF showing pots being filled entirely with clay-balls. Check it out.

It's probably up to you. Each medium is good on its own, and mixing the two in various proportions will probably not make a huge difference.

As for how often to run the drippers - once or twice a day is fine for young plants. 
Plants will need more waterings per day as they get bigger. With a good drip system and growing medium, over-saturation is usually not a problem, as there should be lots of air inside the medium.

The PDF says that even the largest plants won't need more than one (15 min) 'on' period per hour.  For most sizes of indoor plant, I'd say that 4-8  'on' periods per 24hrs  is fine, unless you're growing trees.

Plants will require much less watering in their night cycle; if using 6 'on' periods per 24 hrs, 4-5 in the day and 1-2 at night would be fine.

Google that PDF!