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Proxy servers are good for bypassing censorship, but do very little for security or privacy and also don't reliably hide the fact you are using Tor. If you want to use Tor without it being obvious, then you should look in to Tor bridges with pluggable transports.
Right, pluggable transports are good to obfuscate Tor traffic and in fact they are delivered by 3-rd parties proxies. IMHO, VPN + TOR + obfsproxy is the best bundle for both security and privacy.