You could decide the drive field instability in gravity only occurs at a certain size of drive field, which just happens to be anything larger than a missile.
I'm sorry, but this makes no sense. You are of course free to like how things are now, with missiles from PDCs, but if we do use technobabble at least it has to be consistent
I for one really like this proposal of separating "orbital" fighters, weapons and such from TN fleets/starbases/weapons. It adds in my opinion a very interesting strategic/tactical layer and it is more consistent.
I will also say that it makes more sense regarding missiles to attack planets, when considering a pseudo-science point of view. With max tech, you can design a TN missile that travels at half the speed of light. Supposing a planet with an atmosphere, an item such as this should be obliterated the moment it comes into contact with said atmosphere.
And if we instead postulate that this is not the case because the Tn missile is not really traveling in our "dimension" or something similar, there is still the problem of the impact. A 2 ton missile, for example, impacting the ground at half the speed of light. The kinetic energy released is immense. A warhead would even be unnecessary ...
Yes I know, I'm seeing things my way. But hey, I like this idea and I'm allowed to have a preference
EDIT: In fact, now that I think about it, even just launching such a TN missile from a PDC makes no sense. Even if the TN missile would mostly be "in the other dimension" or something similar, the remaining mass in this dimension should either make it blow up as it tries to leave the atmosphere, or at least have horribly disrupting and damaging consequences on the surrounding cities/landscape.
An item streaking through the atmosphere at half the speed of light? Well, I don't want to think of the possible effects. Hope you don't have fragile things nearby. You know, cities, mountains, oceans, that kind of fragile things.
The more I think about this, the more I like the idea of restricting Tn engines of any kind to operating in space and space alone.