I think everyone who has tried anal sex will agree that if you are hesitant at all, you will not find the experience comfortable or pleasurable. The anal passage is not designed to take objects in so it's very important to make sure you are very relaxed and well prepared. Having said that, if done the right way, you'll be surprised how accommodating the anal passage is, especially with lube.
As much as your friend is keen to try a threesome, I would strongly suggest that you try experimenting with the strap-on just with her. If you are nervous about taking on a 7-8 inch strap-on, perhaps invest in a smaller, skinnier dildo to see how well you can take it. Make sure you have a lot of water-based lubricant on hand and get your friend to gently guide the dildo in. If this works well, then you can move on to the strap-on but I suggest just using it like a dildo also. If the larger size of the strap-on isn't a problem for you either, your friend can strap up. Remember that your friend will not have any sense of how deep she is penetrating you other than seeing how much of the dildo is going in. You'll need to tell her when the sensation is too deep for you.
Once you have explored the strap-on with your friend (and over time developed more trust), you can both look at bringing in the bi dude. You should have no trouble taking in a real penis (don't forget condoms and lube) but unlike your friend, her male friend will be enjoying the sensation of being inside you and may thrust harder and faster. If you are concerned that the guy may go too deep, ask him to wrap a small towel around the base of his penis to prevent this.
As for the extras, I would really consider letting your friend only blindfold you at this stage. It doesn't sound like you know either your friend and her friend that well so being tied up may just have to be a treat reserved for when more trust is built. I would say the same for having your session videotaped.
Finally, don't forget water-based lube!