Cadillac: 1998 catera - oil in antifreeze

I just bought this catera in feb. The head gaskets where just replaced
right before i bought the car. after driveing it for a week or so i
noticed the low coolant light on and when i checked it its clear that
theres also oil in the coolant and after driveing the car for awhile i can
smell coolant. ive been told this could be a bad intake gasket or that the
heads may not of sealed right when they where redone. really not wanting
to have one or the other fixed and find out that didnt fix it. has anyone
else had this problem? ive read many reports on the net sayin this is a
common problem with the catera. if anyone has any advice or know what
exactly the problem is i would appreciate it. Oh and also - after ive put
some miles on new oil the cars oil light will come on and the guage will
start reading lower than normal - just thought i would add that.