I briefly read that Tor hidden services use "Entry guards" in the circuit.
Is there any reason why I shouldn't use the same Tor instance for hosting a Tor hidden service and general internet browsing?
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There's no reason why you shouldn't host a hidden service and browse with Tor normally from the same connection. Be aware that running a Tor relay from the same server as a hidden service can compromise the anonymity of the hidden service.
An attacker that suspects you of running a hidden service could clog your connection (e.g. with a massive reply to an HTTP request) while concurrently monitoring the hidden service. It is always a matter of probability, but for some kind of services it can be pretty efficient.
The attack can also be done the other way: clogging the hidden service while keeping exchanging data with the client.
Compared to relays, Tor instances used for hosting hidden services are much like regular clients used for browsing. They connect with the Tor network only through entry guards (currently three). However, both local observers and entry guards can distinguish Tor instances used for hosting hidden services. Given that they're serving content, download/upload ratios will be different. Also, if many clients are accessing a hidden service, there will be many more circuits, and all three entry guards may be heavily used.
The main point, however, is that it's a bad idea to host hidden services at home or at work, unless it's just a learning exercise.