I'm running Sparrow connected to a public server, with tor proxy enabled. I believe this means servers and mixers can't see my real IP address, but just wanted to confirm here.
Yes. It is answered in the FAQ:
When you configure a proxy in Sparrow, it will be used for all external connections. This excludes IP addresses in the 192.168.*.*, 172.16.*.* and 10.*.*.* ranges, so you can connect directly to your node on the local network even with a proxy configured. If Sparrow cannot connect via the configured proxy, but the server is using an onion address, it will start it’s internal Tor proxy automatically. You can disable this behaviour by closing Sparrow and changing the value of the autoSwitchProxy property in the config file.
So yes. It is used for public servers, whirlpool and mix partners.
are there any issues with running a VPN simultaneously with tor proxy, if I'm only using the proxy for Sparrow?
In general, yes. It is detrimental to your privacy to use VPN and Tor (see
explanation). But if you're using Tor's proxy in the proxy URL, then I believe you'll be fine. Is there any reason to run both simultaneously though?