Stock 6.5" speakers with cassette player

My Z originally came with a cassette player and 5 speaker set up. Are these like the monsoon system where you cant just replace the speakers without having ohm problems? or should I just be able to install aftermarket 6.75" speakers with no problems. Its on a 98 Z28 thanks:D

Also what are some good 6.75" speakers to put in there. I just have a pioneer deck and no amp running the speakers.