I successfully installed Aptana version 2.0.5 plugin in my Rational version 7.5.5.5 iFix1. It seems to work fine so far.
I believe my mistake before was trying to install the latest Aptana version 3.4.2 into Rational 7.5.5.5 iFix1. I don't think Aptana 3 works with Rational 7.5.5.5 iFix1, although it was odd that Rational wouldn't even launch to allow me to uninstall Aptana 3. Not going to try it again though, it takes hours to reinstall Rational!