As far as i know, unless you do an autopsy, the only way to confirm if you have a female is by her laying eggs...
As for are females more placid? Not sure, but an example would be Kev's fahaka on here i think is also a female, but hes said his will tear everything apart that hits the water, but mine on the other hand lives with a rubber lip pleco and a yellow lab cichlid without any real problems.
With that being said, i would guess two things play into this though, 1) being space and decor in the tank & 2) being the nature / personality of said puffer.
Pufferpunk is a good example of this, her fahaka lived with i think a polleni? For some time with no problems, and hers turned on the polleni one day and easentially killed it...
So if you decide to try tankmates like i did (not that i really recommend it), i would try with something cheap you dont mind losing, also add them all in at the same time if possible so they grow up together rather than being introduced when the fish are larger and more aggressive...
Btw, sorry for the wall of text, but thought you'd rather here of peoples personal experiences for more of a better view...
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Okay. Well I don't think I will be performing an autopsy to sex it. However I have the urge to call it a him. I don't feel the need to have any tankmates with him, other than small fish such as tetras. Given that the bio waste will be high already. How do dithers do? Do puffers see them like smaller cichlids do and feel more secure and therefore more active? How do plants do with puffers? I appreciate that it will vary from puffer to puffer. But as a rule?
I really appreciate all your help. You have been extremely useful in aiding my decision.
As it stands now I have him reserved. I will wait and see if I still feel its for me for another week or so before going ahead. The last thing I want to do is rush into this.