I grow in soil and I usaly have about 1'' for the rooting area then maybe 1cm for the stalk and that's all, Hydro will make your plants grow faster but you have to watch the mmp and a bunch of other factors
I've taken off close to 100 clones off of a 2 foot tall plant, it looked bare as heck but it recovered fast , it all depends on how fast they grow so if they grow fast you can take clones more offtin , if it grows slow then you can't take alot of clones off offtin
I believe that all soils with nutes in them are based on there tests and not on how we use it for our growing , so I would have to say flushing the soil would take a good amount of nutes out of the soil