I am installing 3/4 plank flooring in a 4x5 guest bathroom that is adjoined to foyer area that already has wood floor (3/4). Should I start first row at the door or the far wall? When I transition from old to new can I but together or do I need T molding?

Depends on your method. If you're nailing down, you can slip tounge and go the other way, making for less facenailing.

Any way you slice it, start at the door and work toward the wall.