hmm, I always have lots of enemy planets bordering my own and I don't run into much trouble. My main rule is to make sure I never put a Mark IV world on alert early or midgame. on higher difficulties this can kill you. I rather take an uninteresting planet that borders low mark enemy worlds only, than take a valuable target that borders a mark IV world. Also always make sure you kill off most of your threat. Don't just do gate raids, neuter the whole planet while you're at it. Takes time yes, but if you don't have any AI progress over time, it shouldn't matter how long you take to beat the game.
Use tractor turrets, gravity turrets and logistics command stations. This way it will take the enemy a lot of time to get anywhere on your systems. placing just 2 or 3 turrets of each different kind on one planet is usually enough. Will take some time to beat any incoming ships yes, but if you go to the controls option at the lower left bottom of your screen to make sure every planet has 1 or 2 remains rebuilder at all times, they will rebuild any turrets that get destroyed while the enemy ships slowly die. I never have to unlock any mark II turrets until near the end game (well, except tractor and gravity turrets then, I use a lot of them).
hope this helps a bit =)