Beta fish can be put together, but only females. Males and females cannot be put together except when breeding, but amateur Beta breeders do not get amazing results. Males cannot be put together, you're right.
I don't know what you've been reading, but it bothers the fish more if you do NOT get a filter! Your fish will not thrive very well in a tank that is not filtered, because they need a cycled tank in order to thrive, and you cannot do this without cycling the filter as well. You need a heater since Beta's are tropical fish and will thrive in heat, otherwise your fish will undoubtedly be cold and less active.
If you are new to live sand, then I suggest you don't attempt it just yet, and just get gravel since gravel is easiest to clean with a siphon/vacuum. Also, the fine sand may hurt the Beta's fragile fins.
You need an air pump in order to get the most out of your Beta's lifespan which can be 8-10 years if cared for properly. An air pump allows for proper circulation of oxygen within the tank that is being provided by live plants.
Good luck!
~In order for your tank to be suitable for your fish, you need to cycle it. Cycling a tank isn’t that hard, but it does involve patience. Set up your tank in the area of preference, fill it with treated tap water (you can buy water condition in any pet store) and plug the filter and heater (if needed) in. Do not unplug them for five days, since this is the minimal amount of time it takes for a tank to fully cycle.
You should purchase specific bottles which can be sold in most pet stores that are made for testing the ammonia, nitrites, nitrates, and pH. During the cycling process, you should be testing your water daily. You will also see your tank cloud up and slowly become clear again, this will repeat for a little while until your tank becomes stable. When you see your tank water become slightly milky, do not change the water unless you absolutely need to, (like if there is a sudden ammonia rise) or else the bacteria levels in your tank will become unstable. The murkiness of the water is just all the levels of bacteria trying to populate, and settle down.
After you have cycled your tank, you should perform weekly water changes, and replace 25% of the water. In order to do this properly, you should purchase a gravel siphon/vacuum, which is a tool that will help you get your tank as clean as possible and get rid of all the unnecessary waste.