The line is drawn at engine use. Databases are fine, although a "cloud" based DB like Fritz has enters a decidedly grey area and of course you can't use endgame tablebases or ask anyone for advice. There are also folks who simply play the game without assistance of any kind. That's their choice, but your plan of developing your opening theory is sound. Be a little careful though. Sometimes the most popular line has been refuted and the stats simply haven't caught up. Oh and there are lots of videos that can be used legally as well. Have fun!
Correspondence Chess Etiquette
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So, it actually depends on the site: there are some websites that do allow engine use. These can be good for really analyzing in depth every position that occurs over the board. Chess.com, however does not allow any sort of engine, or outside assistance to your specific position.
This also means you cannot call up your GM buddy to see what he thinks about your position.
However, general tools, like opening books, videos, etc, are ok. Chess.com does access to the database during correspondence chess, so what you describes seems perfectly fine.
One of the sort of borderline things might come in endgames: If you have a bishop and a knight vs a lone king, and do not know how to checkmate.....can you look up a video that tells you how? Since the video is not made specifically for your position, I believe the verdict is that, yes it is ok. Correspondence Chess is usually viewed more as a learning tool, with live chess and OTB tournaments the "test of your knowledge", so the rules are a bit more lenient.
I'm relatively new to the world of correspondence type chess. I plan to use it as a tool to devolop my opening theory this would mean during the match I will look up the opening being played and follow a line until the opening is over and the play without assistance. I don't know whether or not this is considered cheating. Any insight unto this topic would be appreciated. Where is the line drawn in correspondence chess?