I recall having these issues trying to set up a server on linux a few months ago. We only had java 6 on the box and it kept giving us errors. It turned out that we needed Java 7. IndustrialCraft required 7 for certain, but I can't recall which others might have.
Is there some explicit reason you don't want to run 7 as well? It can cause some issues with programs that were designed for 6 and set to use 6+, however they don't seem too common and usually when they come up the authors set them to use 6 specifically. If there are no such issues, you should simply be able to install Java 7 alongside Java 6, and use both.