When I was playing Entrenchment, I made heavy use of mines near the warp points of my gravity wells. Also, try very hard to not waste time fighting Pirates. Buying off the Pirates will ensure that another player is tied up and not attacking you. The down side is this allows enemy capital ships to gain experience points since pirates are not usually strong enough to defeat a supported capital ship. IMHO, capital ships should always be supported.
Of course, this requires a good economy. I usually play as TEC since they have the best economy building tech tree. Your home planet is your largest source of income; protect it by establishing a buffer zone ASAP and making sure you upgrade Civilian Infrastructure. Also, research Trade Ports as soon as you can to boost your income. Having a lot of credits on hand will a) allow you to pay off the Pirates, b ) build strong defenses, and c) build and research a stronger fleet.
As far as your question about a balanced fleet, I start with a capital ship that has the Colonize ability. I really like the Akken class because it also has the Targeting Uplink ability which boosts it's supporting ships' stats. I also like long-range frigates and carriers with bombers. Once you have have a decent number of bomber squads, take control of them and start chipping away at enemy capital ships. The AI will usually turn tail and run once they lose enough capital ships.
But, other than opinions on specific ships and tactics, there really is no "formula" for fleet make-up. It just depends on what ships are available to you at the time (research), where you are in the game (beginning, mid, end), and your style of play (offensive, defensive). One thing that will help you decide what ships to build is knowing what your enemy is up to; so, try to keep scouts circulating and make sure they have the Probe ability.