don't uninstall anything. a system with multiple versions of java works just fine. and you don't need to update your environment varables (e.g. java_home, path, etc..).
yes, ireports 3.6.1 needs java 7 (doesn't work with java 8).
all you have to do is edit C:\Program Files\Jaspersoft\iReport-nb-3.6.1\etc\ireport.conf:
# default location of JDK/JRE, can be overridden by using --jdkhome <dir> switch
jdkhome="C:/Program Files/Java/jdk1.7.0_45"
on linux (no spaces and standard file paths) its that much easier. keep your java 8 for other interesting projects...