I'll agree that the missile advantage ought to be dampened, somehow, but I don't think requiring a sensor on missiles is the way to go.
First, EM/Thermal passive sensors can easily be considered light-speed detection. Since ships/missiles are typically much much slower than light speed the 'sneak peck' has minimal impact on the strategic scale, and the tactical significance of the time lag could easily be the reason why passive sensors don't provide targeting solutions.
More importantly, requiring a small sensor on all missiles would actually make a missile offenses stronger since the smallest of all missiles, antimissile missiles, would suffer the most in their roll.
I'd suggest instead making fractional damage weapons for anti-missile defense. Countering increased numbers of fractional damage point defenses would require adding missile armor, which favors larger offensive missiles without penalizing existing anti-missiles.
Here are two fractional damage ideas:
1) Fragmentation Missiles. This replaces the defunct Laser Warhead. A Fragmentation missile might make 2x attacks per point of warhead, each hit doing 0.3-0.5 damage depending on tech level. Alternatively, they might make N attacks at 1/(N+1)th damage, where N increases with tech level. Either way, using fragmentation anti-missiles would be much more effective against unarmored missiles, slightly less effective against well armored missiles, and useless in offensive use against other ships and fighters.
2) Small Gauss Cannons. 2/3rds size (4HS), but each shot only does 0.5 damage. This option provides +50% more shots per HS for your Gauss turrets and CIWS at the cost of losing offensive capabilities.
Also, I'd like to petition that a 0.33x reduced size laser tech be added, with research costs and recharge time across the entire reduced laser tree be reduced to only double that of the equivalent missile launcher miniaturization techs.
