1. what kind of cichlids can i put together?
What kind of cichlids do you want? African or American is a very general place to start. You also should find out about how aggressive each fish you want can be.
2. what other kinds of fish can i keep with cichlids?
Again it depends on which cichlids.
3. for a 150gal tank, what kind and how much equipment will i need?
As with pretty much any tropical FW tank you will need a filter and heater. I highly reccommend getting a larce syphon as it will aid you so much in water changes and tank cleaning.
4. how many fish can a 150 hold?
It depends on the species and size of the fish. You could keep a ton of tetras or guppies but only a few oscars
5. how often do you need to change the water?
Again it depends on the fish. A greater bioload requires more water changes.
I usually change 40% every other week in my tanks.
6. is it best to use live plants, and do the plants require any special care?
Plants have individual needs just like fish do so I would reccommend researching plants that you would like to have. African Cichlids don't really like plants in my experience and it's better to have rocks.
It is best to use live plants as they oxygenate the tank and help to prevent algae.
7. how hard is it to care for a tank that size????
Again it depends alot on what's in the tank but it can be very hard to care for large tanks.
I know that's a lot of ?'s, but thanks for your time