Well... what size tank is this? JW.
EDIT: Wait wait from your signature Im judging its the 55G..right..am I right???
Do you have any tips on picking out a "calm one" ?
Would a female be calmer than a male? Any suggestions?
I have a suggestion for that =)
There are some bettas at the pet store that are flaring and slamming against the tank glass, trying to get at the other fish, etc. Get one that is more laid back, but don't get one that is just sitting at the bottom of the tank. Get a calmer one that only flares rarely and it calmly swimming around the tank.
In
some cases females are calmer than males, but I had a female who loved to rip apart her tank mates. If you get a calmer male (read the "tips" I posted above) then you will be fine, and if you get a calmer female ("tips" for that below) you will also be fine. You might never know if you get a calm male though....because when he sees another fish in the same tank his personaility may turn around. Maybe and then again maybe not. Three of my bettas are in community tanks and they do great. One of them was very fiesty when I got him, but when put into the community he is not as fiest and rather calm now. The other two were fairly calm when I got them in the first place.
Get a female that is not just sitting at the bottom of the tank, but swimming around calmly, not chasing the other fish and not crazily darting around the tank or ramming into the glass "walls". My concerns are below about both keeping a female or a male with a molly....
The only thing I am concerned about with the male is that the molly may pick at his fins because they are so long...very tempting to mollies because they are so colorful and just streaming down from their body..oooh...ohhh....the molly just can't resist, she has to have a FEW picks!!!! Please, let me pick at him, oh please oh please its so tempting

Thats the only thing I am concerned about with the male if you pick a calm one. If you dont pick a calm one....well...then a lot more concerns will come...And then again some females are rather small and tempting to pick at because, no they are not long, but they are colorful...or
most female bettas fins are...then again some are brick brown or dark blue....
Welllp...thats all I have to say

Hope I was at least a
weeeee bit of a help....
