Y'know, instead of all-or-nothing 'cloaking', why not just define a 'stealth tech tree' of five-six levels... Stealth Plating I-II-III, Scanner Interference I-II, Cloaking Device. Stealth Plating would have a rating of 1, Scanner Interference would have 2, Cloaking Device would have 3.
So what would they do? Well, every point of rating would... act as an additional square of distance when determining if you showed up on scanners or not.
So, you're planning an invasion of the enemy homeworld. They typically have ships with a scanner range of 7. You slap two Cloaking Devices (or three Scanner Interference Networks or whatever), and suddenly, they can only see you if you're right next to them.
It would also let you build up fleets without triggering the "Hey, you're building up a fleet! We don't like you!" message.
Size-wise, the modules would need to take up, say, the same space as a weapon (or maybe a weapon and a defense) and would need to be expensive... and the AI would need to be tweaked so that it would occasionally build dedicated Scanner Ships when it believes cloaking tech may be in play... also, this way, cloaked ships aren't really good for defense, since anythign in orbit around your planet is visibl anyway. It's definately a dedication of the ship to offense... but it won't be as good at it as a non-cloaked ship. Once the novelty of it wears off, cloaks may not be useful, so you need to plan out where and when to use them.