I want to convert my old Trek friction/downtube shifting to STI shifters while minimizing cost, so ... keeping the 7 speed rear cassette (or upgrading to 8 if necessary). Already have a 7-speed Shimano 105 rear derailleur in possession. My understanding is that 8-speed shifters will work OK with 7 sp. cassette.
Irregardless of what it takes (e.g. cables, braze-ons, etc.) to actually mount the hardware, can I mix-n-match 7 or 8-speed shifter pairs from 105 up to Dura-Ace with that rear derailleur and likewise can I use a 105 or Ultegra or Dura-Ace front derailleur with whatever STI shifters I end up finding?