A thought, have separate bases for the sensors as for the weapons. That way you can cram more weapon or armor on to your PD bases while still falling under 500 tons. Unlike ships in space, your PDCs simply aren't going to be independently deployed, ever.
And multiple sensor bases, so if one is destroyed, you just have another go active.
I also recommend using a weapon that isn't affected by atmosphere, like mesons.
From an RP perspective, some people avoid putting bases on the planets because they do not want planets targeted with missiles. Others go the poison pill technique, of making planets too expensive to attack, with no reward for the attacker other than a radioactive wasteland.
Others powers have the philosophy of not putting comprehensive fixed defenses on colony planets, because they want the colonies dependent on the navy for security, and therefore willing to pay taxes and minerals to support the navy.
(by comprehensive fixed defenses, I mean they might have point defense, but not long range missiles perhaps, and therefore would depend on the Fleet for that, or otherwise have a range bracket they are ineffective at without Naval support)
