I want to compile my .java's (several) into one .jar that are compatible with at least 1.6 and newer, preferrably 1.5 and newer versions of java. (I have 1.7.0_5)
Yes, you can set the version of compiler at compile time. And compile your java code into old versions of java.
From Oracle article : http://docs.oracle.com/javase/1.5.0/docs/tooldocs/windows/javac.html
Here we use javac to compile code that will run on a 1.4 VM.
% javac -target 1.4 -bootclasspath jdk1.4.2/lib/classes.zip \ -extdirs "" OldCode.java
You might also need following parameter to set denote the version of your code.
-Specifies the version of source code accepted. The following values for release are allowed: