So I think in reality you have to handle only 2 scenarios:
1. Acquisition of new planet
2. Sudden loss of population.
In the first case you don't want too fast fortification (at least I think so :) ) then in my opinion fixed growth could be the best (like 100 battalions every turn up to 1000 for maximum fortification after 10 turns). This can force the player to expand carefully as new planets will be vulnerable.
The second can happen when player looses 3-4 high population planets on the same turn. As the result (s)he has already in use more battalions than (s)he can handle. In that case I wouldn't just drop the number as it can leave player open to attacks everywhere. Instead maybe decay current number of battalions by 5% per turn - or something like that but percentage rather than fixed value. As the result defense on all planets would slowly decrease (probably by that 5% per turn) leaving time to get those planets back.
That way player may avoid nasty effects of sudden drop of defense to 40% on every planet in one turn then having to slowly rebuild it because of fixed mechanic in p.1
Every other scenario shouldn't matter that much as population level shouldn't be that spiky, right?
