My advice is to be very honest with your limitations. My aunt and uncle had 3 girls and wanted a boy and figured why not adopt? They tried foster to adopt at least 2 times, with the boys being taken back both times. I don't know the specifics, but they were upset and my cousins found it hard, too. My aunt and uncle wanted a boy of any race, under 4, with no major health issues (minor were fine). They eventually decided that their hearts needed a certainty so they went overseas and adopted my cousin. It was a longer wait, but they had more certainty about things (like his parents' rights already being terminated).
I've also talked to a mom in our playgroup whose son is adopted and she says so many will say to the birth parents "Oh, I'll keep in touch, I'll do this or that" knowing that they likely legally won't have to - which is hard on the birthparents. Just be upfront. If you can't take a child with major needs, then don't. If you can't take a child where the parents want updates, then don't.
My grandparents fostered and my mom says even now she wonders about some of the kids that went back and they never heard from again. If you get into fostering, make sure your girls get whatever assurances or resources they need to, especially if they are likely to have several foster siblings for short times.
So...good luck and thank you for giving a child a family.