There are only 2 types of gators ... americans and chinese, I highly doubt you will ever see a chinese available in the pet trade. Chinese gators are also quit aggressive.
Dwarf Caimans (P. palpebrosus) which get about 3ft-5ft, Smoothfront Caiman (P. trigonatus) which get about 4ft-6ft, Spec Caimans (C. c. crocodilus) which get about 4ft-7ft, Hybrid Caimans (crocodilus x yacare) which I think would get about 4ft-7ft ..... these are whats the smallest species that can be found in the pet trade. .... these all have typical croc attitudes, the dwarfs and smoothfronts being more of a shy animal trying to stay away from you but will get fiesty when and if handled or if you get to close to them.
Dwarf Crocodiles which get about 4ft-6ft can be found once in a while in certain states but they are pricey. These are about the same as the Dwarf and Smoothfront Caimans, my adults tend to get a bit aggressive at times but nothing big.
New Guinea crocodiles are available from time to time, these get about 7ft-8ft, and are actually quit docile, also a slender crocodile for the most part if not over fed ... these are still a crocodile and not to be under estimated but these are the most docile species I have worked with, these however I find to be very aggressive/testritorial towards other crocodiles (male or female) ... and again pricey.
I think your first concern should be housing of the animal, not its disposition, even if a species is considered docile or aggressive as a whole, each individule animal does have its own personality which can be more docile or more aggressive.
I keep 11 or 12 differant species of crocodilian and to me there all just croc's, I dont really see them as aggressive or docile .. I just see them as croc's each with its own personality. They are a hands off species as far as im concerned so as lond as care is taken it really doesnt matter which is more aggressive or more docile, they should only be handled for enclosure purposes (cleaning, maint, etc).
Hope this helps