Oh Julia!! I can't get on FB from work but I wanted to answer your questions!!!!!
#1 - Youth hostels are nearly unheard of in LA except in the worst parts of town. Of course the problem with asking me is that I'm a bit of a hotel snob and can't stand the thought of a hostel. Out of those I have researched probably the best would be the Hosteling International location in Santa Monica. Fairly safe area with good access to all major points of interest.
#2 - Don't bother buying Disney tickets in advance. There really are no discounts except for multi-day passes through tour companies and those generally require you to book hotel in conjunction so you would end up spending more. I can say that you should definitely visit both parks if you can, but if you are limited to only one then definitely do Disneyland, not California Adventure. It is the original, the only park that Walt actually walked in and spent time in. It gives it a little something extra. Of course, I'm biased in that respect since I'm from SoCal. LOL.
#3 - I assume you are not planning on renting a car while you are there? Oh wait! I forgot! Are you still not driving? It is not easy to get around SoCal without a car. Pretty expensive proposition actually. Taxi's are ridiculous especially since you spend a lot of time sitting in traffic and being charged for every minute of it. There are some shuttle services that will transport between major attractions and the airport. Super Shuttle and Prime Time Shuttle come to mind. They are door to door shared ride service but not cheap. Public transportation is nearly impossible because lines don't always connect to each other without going far out of your way. I would suggest checking with the Los Angeles Visitors Bureau and see if you can plan out your routs using the metrorail and other public transport. Anaheim (where Disneyland is) is in Orange County, about 35 miles south of Los Angeles.
I don't know if any of this is helpful or stuff you have already found out on your own. I hope you can use it to further your research. Departure date is coming up quick!!!!
Enjoy every minute of your time away!!!!!
T19 M15 S19 XH47 M43 bomb12/4/07 PA5/07 S12/26/07 D final 11/17/08 Back together with no defined R 05/2010 confused....to say the least!!!