Even though they found no crohns, I would be very suspicious of there findings. Rectal fistulas don't just happen for no reason. You have something going on in your colon that is causing that inflammation and abcesses. Surgery is only a temporar fix because if you don't stop the underlying inflammation, the fistulas will just resurface.
In 2007, I developed two massive rectal fistulas which started out as small abcessess about an inch from my anos and finally burst open into full blown painfull fistulas. The surgeon put in two setons two help them drain and told me that I would probably have these the rest of my life. In my case, they did find crohns in my lower colon.
The GI told me that fistulas as big as I had will never NEVER heal on there own, and the only way to close them was for me to go on remcade, which I refused.
I did my own research and I decided that I would try the SCD diet. The followed the diet with 100% fanatical dedication, and after 4 months, the first fistula was so healed up, the surgeon had to remove the seton so it could heal up completely. By 6 months, the other fistula was healed also. That was in 2007 and to this day, I have not had a problem or symptom or a fistula from my cronhs. The only drug I take is pentassa which is very mild drug which has no side bad effects.
I suggest you read the book "breaking the vicious cycle" which goes into detail about the SCD diet. You have nothing to loose by trying it , and all to gain if it works for you.