"..........Today, Ive upgraded to.......Bezzera BZ99 and a Rocky grinder. I was surprised that Rocky was capable of choking the bz99 at setting 9-12, and needed to be at around 15-18 to get correct extraction. Is this normal?........"
You find the correct setting for the type of blend VS grinder VS machine. The numbering *scale* in any grinder is arbitrary..it doesnt mean anything...having written this however, most grinders have their "scale" set in the factory so that zero, is where the plates basically rub together. After that, its anyones guess, and whatever number you end up on for your application is...whatever it is.
We generally find that a setting of *around* 15ish on a rocky, is about right for commercial *style* pump driven esp machines...so you are about in the right ball park.
".........I also noticed that my gaggia filter basket passes water more freely then the bezzera filter basket. Which would be better to use?
You have moved to the BZ99. Therefore use the filters it came with OR, use generic commercial type replacement filters suitable for the machine. *OR, it may be the BZ99 came equipped with the *smaller* Italian type filters intended for use with ristretto type coffees...they might be *smaller than those provided with the gaggia even though it is only a domestic machine, but intended for this market where people drink bigger coffees! *This actually would result in the reverse of what you have apparently observed ( the bigger filter should flow slower). In any case the filters between these 2 machines are basically interchangeable so I would use the bigger of the two types, and dont worry about the different flow rates you appear to have observed. Simply adjust your technique /grind to use the filter of your choice to get the desired flow rate with the BZ99.
".........I find the steamer on the Bezzera often boils the milk before I can get a nice foam happening. I think I need to turn it down a notch..."
The steamer does not *boil* the milk. The operator does! *;D This is normal given you have just moved from a machine with tiny capacity to a semi commercial type of much bigger capacity, and you will have to adjust your technique accordingly! *;) Others Im sure will come in with helpful hints on technique however in order to save a lot of duplication it may be as well to mention this has been very well covered already in the "milk froth and bubbles" section above in the last couple of days, see the link:
Hope that helps.
The Bz99 is a nice little machine. Learn how to get best advantage from it and..............enjoy.