Are you running the most recent version of cal? Maybe you want to download the current development snapshot - there's a real good chance that your problem is already solved, even if it's not commited to TER yet.
In case that this doesn't work for you, please have a look into our bugtracker to see if it already has been reported. If not, please come around into the newsgroup and tell us a bit more details. What's running wrong? What other extensions do you use? What's your TYPO3 version?
Q: How can I automatically fill the enddate field with startdate when creating events in the backend?
A: Simply hide the enddate field. cal is filling the enddate automatically if it's blank. At least for current development versions this works also if you change the start date somewhen later.
Q: How can I extend the list of available category stylings such as green_catheader, ...?
A: Edit the page where your categories are stored and put something like the following TS code there:
options.tx_cal_controller.headerStyles = default_catheader=#557CA3,green_catheader=#53A062,custom1_catheader=#FF6D3B
options.tx_cal_controller.bodyStyles = default_catbody=#6699CC,green_catbody=#4FC464,custom1_catbody=#FF6D3B
Maybe you also want to adjust your CSS styling.