*well, you SHOULD have at least 2gallons with a hang on the back or sponge filter and a heater and only need to change 20% of the water each week using a siphon or gravel vacuum. however... if you don't have a filter you'll need to change all the water once week. do so by:
1. putting the betta in a cup of half new water half old water (I used to use the cup they came in).
2. dump out all the water and put the rocks in a strainer and swish around under running water.
3. wipe down tank and put everything back in.
4. fill with clean conditioned water as close the temp of the old water as possible, put heater back in and float the betta in the new tank until his cup and the tank are all the same and correct temp.
*as said above, you SHOULD be able to keep them in a tank because you should be using a gravel vacuum or siphon. however, if not put them in a cup or bowl.
*depends what other fish and how large your tank is... pretty much NO other bettas, no other fish in anything under 5gallons or more.
*males generally have longer fins... however some males have short fins. females will have a white dot on their bellies and males will have a big black beard at their gills that they puff out when you put a mirror next them (females have much smaller beards)
*noooo... this NEVER works. it may seem okay for a while but someone will ALWAYS end up hurt. for it to work you need upwards of 50galloons and heavily planted.
*YES. you ALWAYS need conditioner to be 100% sure the water is safe (even well water, and places where no chlorine or chloramines are added... things can get into the water that are fine for us but could kill your fish and you'd never know until it was too late). $3 bottle treat 100gallons. no need to get the betta specific stuff, and freshwater conditioner will do. aquarium salt is also something to look into.
here, check out this link:
http://www.ehow.com/how_4489425_properly-house-betta.html