3 random questions

1. will too large of a exhaust pipe cause erratic idling such as idling too low(below 1000rpm)?currently running 2.5" all the way back but it seems to idle low after i did this, think i screwed up the back pressure or something.

2. my civic doesnt smoke when idling or up to 4000 rpm but if you gun it it blows blackish gray smoke out it's rear(love to get in front of mustang convertibles like this) ? Is this a sign that my rings are shot, the headgasket going, or could it be that the engine is running rich? I havent done a compression check yet

3. what is the best rebuild kit to buy for a d16z6, i have a motor for my civic but it spun a crank. will all the parts (i.e. alternator and such) fit on the d16z6 off of my d15b. is the d16 and d15's transmission compatible with each other. why is it that the dx is always shown without a tach and mine has one....

i realize i am acting like a 12 year old asking this but in the past year i forgot alot not being around civilian cars, i just need a refresher course