Nicki - I totally understand your fear, it is one of my biggest ones too. Having had quite a long time to process all of these thoughts now I have come to the conclusion that I will always be scared, no matter how long I wait or what I do. It is totally natural to feel this way after such a traumatic experience.
With regards to the folic acid, I started taking mine as soon as my hcg hit 0, I had to wait until then because of the metho shot I had and the risk of the folic acid interfering with it if I started taking it earlier. I was very scared when I first started taking it that it might make something grow again (stupid I know) that would give me problems, but nothing bad as happened and now the way I think about it is that it can only be a good thing having all the extra vitamins floating around in my system. I try to put the bad thoughts out of my head as they tend to overtake all my rational thoughts. Statistically a second ectopic pregnancy only has a very slightly higher chance of happening than if you've never had one, it is after you've had two that your chance jumps up significantly, so looking at it that way neither of us has a much increased risk of this happening again as we did before we even got our bfp's last time.
Hope that helps a bit, sorry I've rambled on a tad! Xx