I'll be headed to Croatia this summer for a short getaway - I'll be flying into Zadar, then going onto Split and finally to Dubrovnik, before heading to London. I will be arriving in Zadar on 24 June at 8pm, and heading out from Dubrovnik on 29 June at 11am.

Some questions:
Does anyone have information with regards to the bus from Zadar to Split? Are there any buses operating after 8pm? And how long does it take from the Zadar airport to the bus terminal? I've looked at http://www.akz.hr/EN/voznired.aspx but I'm still a little unsure. How reliable are the bus timings? Alternatively, anyone stayed in Zadar before? I'm thinking of heading straight to Split because I don't see anything of much interest in Zadar. I'm tentatively spending 2 nights in Split and 2 nights in Dubrovnik.

Also, for the bus tickets, must I book in advance (is there even any website to do this?) or is purchasing on the day itself okay?

Finally, I'll be travelling with just another girl friend, and both of us are rather petite and young-looking (people often mistake us for teenagers) - any advice on safety and the like? Any help will be appreciated, thanks!

no one who has gone to croatia?

No worries about safety--I have visited the country three times. Just take the normal common sense you would anywhere. No need to buy your bus tickets in advance (in high season like August I would recommend on some routes).
There is a bus from the airport to the bus station and old town, see schedule here:
http://www.zadar-airport.hr/en/kako-do-nas.php
It is a low season schedule, so can only increase in June.
As you see on the bus schedule on the link you posted, there are buses at 22:30 and 22:45, so I'm sure you could make it if you don't mind arriving in Split that late (near midnight). You should have your accommodation in Split lined up in advance in that case.
Zadar is pleasant if you want to spend a night there. There are some interesting sights you could see in the morning before taking off.
Croatia is easy to travel in--have a great time!

As Yorhshire suggests,I would spend some time in Zadar.

just be careful in bars ...dont know if that dubrovnik killer has been nabbed
dont go anywhere with locals
and have your wits about you around dubrovnik if youre walking out of the old town
otherwise safe
i travelled with OH...we had a car and we walked a lot and saw no trouble but then we werent out at 2am at salsa bars

I was there alone last year for about a week, split, trogir and the majority of the time in dubrovnik, had no problems and travel was pretty easy. If you get a chance you may want to take a day tour to Montenegro- it is spectacular!! Have fun!