To the best of my knowledge, this is what is known about interdiction effects in the game and how they stack.

Slows that affect speed and turning:
- Interdiction Drone -- 40%/50%(T4R)
- Interdiction Mine -- 50%/60%(T5L) 15s/20s(T3)
- Interdiction Missile -- 50% 8s/10s(T3)
- Interdiction Drive -- 20%/30%(T3L) 6s/9s(T3R)

Slows that affect speed, but not turning:
- Ion Railgun Slow debuff (T5L) -- 55%(tooltip)/40%(in-game) 12s(tooltip)/6s(in-game)
- Concussion Missile slow debuff (T5L) -- 35% 15s
- Ion Missile slow debuff (T5L) -- 40% 6s
- Siesmic Mine slow debuff (T4L) -- 50% 3s

Slows that affect turning, but not speed:
- Servo Jammer -- 20% 20s

- Sabotage Probe -- 100% 6s/7s(T2) -- maneuvering, which suggests turning, but not speed
- Sabotage Probe slow debuff (T5L) -- 35% 7s(T2) -- speed only, this is the bugged upgrade, which disables the maneuvering disable

Slows in the 1st 2 groups stack with slows in the other group, but not with those in the same group.
How the remaining slows stack has not been tested.

References: -- build calculator for stats -- Drako's 1st post -- Drako's 2nd post -- Verain asking about Servo Jammer, but no answer -- Drako's testing of ion railgun T5 slow, see the rest of the thread for context and why it's believed to be 40% -- Sabotage Probe bug