so am i gona probaly blow my speakers if i just keep gains at 0
I personally would bridge the amp, and use your head when setting the gain and using the volume knob. If the speakers sound like they are being stressed, back it off some more. You should hear warning signals from the speakers before damage occurs, if it will. But using the volume knob and amplifier
intelligently, you should run into no problems.
but my question is if my friend just screws in the tweets and the 6.5's will they sound good or do they need a custom install?
It depends on 1) your friends definition of "good", and 2) the details of the door itself. Results will probably be poor if he just screws them into a door card or thin plastic mounting support. The mid will need to be mounted to a solid baffle (i.e. if there isn't one, he would be doing himself a favor to make one), if mounting into the door it would also be beneficial to sound deaden the door and seal the door to separate the backwave of the speaker from the frontwave. All of this can be done relatively inexpensively if he's atleast decent with even a jigsaw (or knows someone who is) and has some help choosing sound deadening materials and creating a deadening "plan" (I would recommend reading
http://www.sounddeadenershowdown.com ).
As for the tweeter, it's mostly about install location and aiming. Just grab some strong double sided tape and play around with different tweeter positions. IF he choose to do something like mounting them on his A-pillars (not that I recommend it personally, but some people like it that way) there are cheap and easy ways to go about it that won't take much time or money. He won't need to go crazy with fiberglass and bondo.
He'll be much more pleased with the results if he takes his time and does it right and spends a little bit of extra //content.invisioncic.com/y282845/emoticons/greedy.gif.5a53e6246569d7ab79867170f3b06629.gif on deadening materials rather than just tossing them in and hoping for the best.