First of all SSB, I'm so sorry. Clearly, you are feeling awful about all this and are totally drained.
It was wise of you to ask for help for you and your six year old daughter. You have not failed your daughter. As you can see, this is an entire website devoted to help parents concerned about their kids exhibiting behaviors similar to the ones you have mentioned. Often times, there is some sort of genetic predisposition that causes our children to behave differently. Proper medication, behavioral interventions and they say parents who learn how to react in a calm, self assured manner...make the difference.
Sure, this is a lot to do and one thing that I find a tad frustrating is that it is hard to find the resources, but all starts with asking and that is what you have done!
It does sound like your daughter's biological father might have some sort of issues that were never addressed appropriately. Was he ever diagnosis'd with anything? Even if you don't know what that is exactly, in the back of your mind, surely you realize there is a chance that your daughter's father has something going on...ADHD, BiPolar (BP) Disorder...but it really could be anything.
If the bio dad is showing signs of not being "stable" for example, shows up to pick up his daughter drunk or is cussing, then I wouldn't let you daughter spend the weekend with him. If he does this a lot, I would get an attorney and stop the visitation.
You mentioned the big delay in getting her evaluated by a behavioral specialist. Why is that? Can you get her to a child psychiatrist? A child psychologist? You know what, I would get the ball rolling...PERIOD. I would go to the next town if I had to, but I do think you really need to start moving on this. A neuro-psychiatric evaluation is good, but might require a specialist that is expensive and has a long waiting list. You can do this later...in the mean time, you could get her to a child psychiatrist to see if they have some ideas. There is nothing wrong with doing a few different things.
You absolutely have not failed your daughter. You have a LOT on your plate. A lot of things to take care of.
Sometimes when a lot of things pile up, its hard to spread the attention around, but as best as you are able, please make sure you give your six year old equal mom "time." You mentioned an older daughter with special needs and she might be taking a lot of time away from the younger one, or at least it feels that way to her.
Are there any kind relatives that might be able to help you? An aunt or grandmother who can come over and watch tv with the kids or take the to McDonalds, etc. Just an hour of something fun and loving might be a real plus for all concerned.
Meanwhile, as best as you are able, please take good care of yourself and your relationship with your new husband.