S.D.
I agree with everyone's advice regarding make-up. I think it's okay to allow her to wear light lip gloss, blush, and a little mascara. Teach her how to do it simply and tastefully, along with the concept that "less is more". In middle school, so many girls are already wearing make-up, some of it being way too heavy. She just wants to fit in, feel a little less like a little girl, and dabble in more grown-up things since she is growing up. Just give her the guidance she needs to not over-do it and to remember that her inner beauty is more important than her outer beauty.
As for thongs and boys, these questions are a little more concerning just because of how some girls use their limited knowledge about sexuality to attract attention from boys. Some girls, NOT ALL, may try to "show off" their thong to get attention from boys. If you think her interest in thongs is more about just being like her girlfriends at sleepovers, then look for some girlie alternatives to thongs that you're comfortable with. I think the suggestions that other moms gave as alternatives to thongs were great ones. Any girl at her age who is allowed time with boys needs to be supervised. She may be totally trustworthy in how she will conduct herself around boys, but she may not know how to handle some situations that could present themselves. Some boys at the same age as your daughter are beginning to experience their hormones and are becoming more interested in sexuality. Some boys will test girls to see what they can get away with...either through suggestive comments or "accidental" touches on a girl's body. Girls need guidance on how to deal with those kinds of situations so that they don't compromise their self-esteem.
Bottom line: your daughter's questions are also her way of exploring what your values are and to "test" what limits you are going to set for her. As much as she may protest or complain sometimes, kids want and need limits. Explore your own values first and then see what freedoms you're willing to give her because she needs opportunities to explore new things while also being guided within boundaries. For example, if you're okay with thongs, then help her select ones that still allow her to be modest and maintain her youth. It sounds like you're doing a great job of creating an environment where your daughter will ask questions and let you know what she's thinking about. Keep that up and try not to over-react if/when she asks you or tells you something unexpected or "shocking". Ask her open-ended questions to find out what she thinks about things too, which will help her learn how to evaluate her decisions.
Your daughter is very lucky to have a mother who cares enough about her to seek out advice. Best wishes to you and your daughter as she blossoms into a beautiful young lady.