Great answers above. Start low and work up. The only absolutes for the components are the calibre, bullet weight and powder type. You can use different brass, primers and bullet brand or type as long as you use the right powder and bullet weight and start at the bottom loads and work up for your rifle.
Quite often I will try say a new 308 Winchester 168gr BTHP jacketed bullet with Varget, I don't have data for this new BTHP bullet but will use Speer or Lymans 168gr 308 Win bullet data with Varget and start at the bottom and work up.
When using very different bullet types cast vs jacketed, or normal lead/copper jacketed vs Barnes copper solid then it is better to go to data for that specific bullet type as well.
All that being said, if I worked up to absolute max for my rifle with say Varget, Win Case, Win Primer, Sierra bullet and wanted to try the Hornady bullet or the CCI primer or Lapua case etc I would back the powder charge down to a lower charge and work up again after changing any one component. Even getting a new batch of powder from a different manufacturing lot could make a difference if at max already with old batch of powder.
