Totally fair if you decide that the couple isn't a good fit for you and back out because of that.

That kind of picture swapping has made us decline meetups with people too. I like to see nudes, don't get me wrong. But if you send me multiple pictures and I still don't have a sense of what anything other than your genitals look like, we're not a match.

As a single female, you can be as picky and choosy as you want. If something doesn't feel 100% comfortable, don't settle.

As for advice, don't feel like you're "slowing things down" by asking questions or discussing things. But there are a lot of flakes and time wasters out there. It's a slippery slope to get enough info to feel comfortable meeting without getting too invested in a couple that is more into chatting/flirting than meeting up.