In the AS7 configuration file you can specify modules to be started using the following config:

<module identifier="org.apache.felix.log" startlevel="1"/>
<module identifier="org.jboss.osgi.logging" startlevel="1"/>

However the order in which modules are started within a startlevel is random. It would be better to adhere to the order specified in the configuration as some bundles can be picky about this. This generally relates to legacy bundles that weren't developed with OSGi in mind.
Although this can in theory be handled with start levels it's often more practical to keep the start order within a single start level too because otherwise you may end up having to introduce a large amount of startlevels in your code and this may not be what the users wants to use startlevels for (startlevels are often used to define the available feature set instead, like on Linux).