Not yet, I'm using a Linux distro myself so I wouldn't know about using your own ISOs - highly doubtful.
Their FAQ (click FAQ on the page https://www.hetzner.com/cloud/ ) says this: Snapshots at Hetzner cost € 0.0100/GB per month (cf. above page after switching language to USA as well the pricing/VAT to USA) [for clients from USA, otherwise change accordingly].
You're right that 1ms latency is more important for automated trading where the order submission is on the server. For remotely chart trading through a VPS, the latency is less important, and instead it is important to have a strong server that won't feel laggy. All of our VPS have dedicated CPU cores, meaning that chart trading over VPS feels just like trading locally, without any freezing. Not everyone chart trades full time at home all day. Many of our customers use their phones during the morning sessions while at work to scalp for 30-60 minutes while on break. Also for traders waiting for specific scalping entries, it's nice to be able to walk away from your desk, switch to mobile while running errands, and still be able to monitor the market to not miss the trade setup.