As far as tactics for Imperials - One is to use your prow mounted torps to cause Chaos ships to turn away before they get to fire their prow lances at you. Torps are great for channeling your opponent where you want him to go. I also like to use 4-6 ship escort squadrons in base to base contact so I can really put out a large torpedo salvo! Always protect your stern. Your ships are slower than Chaos or Eldar, but this can be used to bait a trap for the faster ships. Get them to come to you and keep a staggered line so you have ships withheld to respond when they try to cross your T.
For a bait and trap trick, have one of your ships allow itself to get pursued by your opponent's fleet. When it gets close enough to your fleet, use All Ahead Full orders to speed it ahead into your own fleet lying in wait, and use AHF orders to move your fleet forward and launch a massive group of torpedo salvos toward the opposing fleet. Since there will be around six torps per ship, the threat of the incoming torps will usually cause the opposing fleet to attempt to break off its pursuit, resulting in a mess of ships trying to get out of the way of the torps and the rest of their own fleet. Then just sail in amongst them and wreak havoc with broadsides! But make sure you avoid your own torps!
If you are using Nova Cannons, hope your opponent goes first, because this will mean that when it comes time for you to fire, you will already have a good idea of the range for your Nova Cannon!