I'm gunna go against what's been said. There's two rules that are pretty much universal (although there's exceptions to everything).
1) Don't ask people out on dates, while they're on the clock. Basic stuff.
2) If a woman is nice to you (a dude) while working, and she works in the service industry, it's because she's being payed to be nice. In my experience, café staff, wait staff etc, especially the women, get hit on enough that if they genuinely think you're cute, they'll initiate a conversation beyond the role they're being payed to do. Because you're also on the spectrum, I'll clarify that small talk is not that.
However, being friendly is free, and you never know for sure. Just don't be That Customer.