I need new front rotors on my 04 GT and im going to replace them with slotted ones. Is it ok to have front slotted rotors and rear stock ones ?

 

That's perfectly fine. Most of your stopping power goes to the front anyway, and your ABS will work out any of the locking up issues if there are any.

For what it's worth, slotted/drilled rotors are much more of an appearance upgrade than performance. They dissipate heat faster, because there is more surface area due to the dimpling/slotting, but they also do not have as much mass, and therefore fade quicker. I'm not sure that in a street application there will be any noticeable difference either way, but on a dedicated race car I'd go with blanks.