Actually just looking over your tank stockings :1 male betta =2"X1 =2gallons needed.5 silver tipped tetras =2"X5 = 10gallons needed.so if you need 2gallons for the betta and 10 for the tetrasin total you need 12 gallons to house though, i would not recommend getting anymore.Here is a good website full of information on bettas, since you seem to not be very well informed, please read this then ask all your questions you may still have.http/www.fishpondinfo.com/betta.htm
The one inch per gallon rule is useful sometimes, but it's a silly rule a lot of the time .
Provided actual space for the fish to feel comfortable and good water stats there's no problem with discarding it.
I strongly recommend he posts his water stats here before adding the new fish though.