First, from the LA Times Correction Section:
Simi Valley mall - A photo caption in some editions of the Oct. 24 California section with an article about a mall opening in Simi Valley said workers were planting cactuses at the site. They were planting agaves.
Now to answer the second question, first, a cactus is always a succulent as all cacti are succulents, but a succulent is only a cactus - when it is a cactus and not another kind of succulent. An agave, thus, is always a succulent but never a... cactus. Too bad the Times in their correction did not take this opportunity to education their readers on this commonly made error.
Now as for the plural of cactus; the preferred and more correct word is cacti, though with the rapidly declining standards of the proper usage of the Latin/English terms in the popular (and not so popular, i.e. - the LA Times) press, the word 'cactuses' has reared its ugly head. However, even in these increasingly debased times in which we are forced to live, our Lord-God GOOGLE agrees that cacti if the preferred term by a margin of 3,171,000 to only 174,000 for the term the LA Times so mistakenly uses.