another quiestion

[Nitram Draw]

Remember, if you try to defend everything you end up defending nothing as you will go broke.
Units are definately the best planet defense.
Early in the game mines on planets can be effective for protection, they are cheap and don't take up much storage space.
Fighters become a better option later in the game but can be countered by a few dedicated PD ships.
Sats are good in worm hole defense but are not so good defending planets as you usually are on ther wrong side of the planet to defend.
Bases are good. They pay half maintenance and have extended weapons range.
WP are also good, espescially if you play with the mod that increases their range too, but they take up a lot of space so you usually don't have many except on the green cross planets. They also re usually more expensive to build.
Like SJ said defend the wormholes to your system and use a variety of things. You really don't need a big fleet, in fact most of my fleets have more support ships than attack ships. Rarely do I have more than 25 attack ships at any one time. Build wisely and you can get by with a lot less.