There are some really excellent FPs from Bernd on this, Karel. Finding them is another matter. The new site will make this easier and the search function will be working again.
My answers as someone who is certainly not an expert in Asp but has spend quite some time fishing for them in Hungary.
1) 9’ #6 is fine
2) mostly floating line for me. In fact I’ve only caught them on floating lines. But I’m sure sinking lines will work too. An intermediate could certainly be handy.
3) I never use fluoro. 8lb mono is fine. The diameter of mono is more related to the size of the flies than anything else.
4) Gurgler white, white zonker, bunny cat. But you might want to get more specific with imitating the baitfish if that’s not working.
5) they seem to hang around for a little bit and I’ve caught them considerably after the splash. The way I watch them feed is they hang around, or disappear. The bait settle and then they chase the bait up to the surface. Smash smash. Then they go back to chilling out. It’s about fishing where the bait are hanging out.
6) big asp are louder!
7) some suggest very fast. But I think you need to vary it. Usually I start slow, using the current and then speed up. You want to give the flies time to sink (if not a gurgler). Fish the seams.
I’ve often used two flies. A gurgler and a cat’s whisker. I really like the gurgler because it’s visual.
How big are your asp?
Cheers, Paul