keeping the tweeter should be ok BUT it may drop the OHM load of the speaker down...

what you should do is see where the tweeter is connected...then go to the harness and find out what the ohm load is of the factory front speakers PLUS tweeter...

then connect the pioneers...check the OHM load of both the tweeter and the pioneer combined...if it is any LOWER then 3.3 ohms DO NOT leave the tweeter connect...

hope that helps...
if it wasnt clear for you let me know ill explain it again...