You'll definitely want to go with either male or female, not both. Assuming you're talking about your normal store-bought Betta Spendens and not a wild betta species, males and females can't be kept together in the same undivided tank as long-term tank mates except in very special circumstances by very experienced breeders with sibling fish they have personally bred.
You could certainly do one male, one female, OR a group of female bettas called a Sorority(http://www.bettafish.com/showthread.php?t=29402
<-- Link to the forums sticky on Sororities, and there is tons of awesome Sorority info if you search the forum)if you like though. All up to your preferences.
Platys could be a hit or miss, it depends on the betta. You might have a little more luck with them if you went with a single female, an aggressive male might not take too kindly to them....but a calmer male might do just fine. Its hard to say. If you go with a Sorority, I wouldn't add them though because you'll need the extra room for the proper number of girls.
I personally adore Rasboras, the ones I had way back when were very peaceful and just totally adorable. A nice school of them with a single male or female or even a Sorority would be just fine I'd think.
A few Kulis should be alright since they tend to stick to the bottom and keep out of the way. If you do a Sorority along with a school of Rasboras though not sure if that'd be too much stock-wise....
Best of luck!