it's a fireball of a question. but i'd definitely recommend against using a rear child seat for an 8-week old. search through the forum archives; those that introduce their children to bicycles prior to 12-months-old almost always use trailers. there are good reasons for this, most of which are fairly obvious/apparent. some have had varying success with mounting a car seat into a trailer (also search the forums and Google for this), depending on the car seat and the trailer.
as for conventional wisdom (and the fireball portion of your question), some will applaud your choice, and some will vent all manner of disapproval. there are numerous websites weighing the pros and cons of both sides of the argument, and worth reading so you can make an informed decision.