Off the top of my head I don't know what ammo Spire Teleporting Leeches use, but the other two teleporters use Minor Electric, which most structures already have immunity to. Also, teleporters die in droves anyway against even minor turret defenses, and I think most(?) command stations are immune to minor electric (if not, they should be made immune as a result of this change).
When you say "allow them to teleport in, but make their shots still hit the FF," do you mean that even if they teleport into the FF, their shots will still hit the FF instead of targets inside the FF? That doesn't really sit logically with me, since a forcefield is a bubble, not a completely filled-in sphere, so being inside the FF should allow shooting at targets inside the FF without problem, because you've bypassed the bubble.