Spark 1.x supports the following command line options (should also work with Spark 2.5.6):
open a p2p chat window
open and enter a MUC
So one can start Spark on Wndows with "Spark.exe username=Matt&password=Kitty2007&server=example.com&start_chat_jid=Gato@email@example.com"
It should be possible to start more than one p2p chat and enter more than one room but one should investigate first how this works before adding it here.
"%USERPROFILE%\Spark\spark.properties" (on Windows) stores a lot of settings one may want to modify after installation for a more or less automated setup without user interaction. One can also store there "username=" and "server=" instead of using the startup options.
Remark for those with edit rights, visible for everyone: If you find an error here or want to add some more options feel free to do this instead of writing a comment below the document about things which could be done better.