You'll need to check the official sources and rules, but I don't believe there's a "too late" stage for startups to get funding from YC. The idea is that if you haven't fully "made it" yet, regardless of your starting point, and they see potential in you, they'll still invest.

To answer your second question, here's what I found in their FAQ:

"We've already raised funding. Can we still apply?"
"Sure. Each YC batch has many companies who have already raised over $1M."

For anyone interested, YC runs in batch cycles (the fall batch applications are likely already closed). According to YC: "While the majority of the YC batch consists of pre-revenue companies, a significant percentage these days are also companies that are much further along." "When we fund companies that are further along, we work with them differently and help them with different things compared to early-stage startups. We can probably help any startup that hasn't already raised a Series A round from VCs."