A word of caution about Advantix though, it is poisonous to cats, so probably not a good idea if you have so many cats around.
I use the Sentinal Spectrum one, not sure what the difference is between that and Interceptor. I think Sentinal prevents tapeworm and the other doesn't. If I were you, that is what I would be more concerned about than heartworm as they can get tapeworm from fleas.
As someone else has suggested, you should ring the shelter to find out exactly what they gave so you are not doubling up