The advantages of the Marine Battalion are that it can split into Marine Companies and both Marine units are very effective in fighting boarding actions against ships and PDCs.
Ok, I've only built two Marine Companies so far, so I might just be missing the mechanism, but I don't see where this split/unsplit is coded up. Whether or not it is, I have the following suggestions:
1) Introduce a Marine Battalion HQ unit that can have 4 companies subordinate to it. This would allow the companies to be grouped on the ground unit screen and appear like a Marine Battalion unit. The stats would have to be adjusted to get the combat power right for 4 companies + HQ = battalion.
2) Allow a Marine Battalion HQ to be one of the units subordinate to a Bridge HQ. That way it wouldn't matter if you split or unsplit.
3) If you haven't coded up the split/merge code, you could have it be an operation like "convert to cadre" that can be done on the Battalion HQ (and would require 4 subordinate companies).
4) I think I read that engineering regiments were 4 battalions in size. Why not just make them engineering brigades and make them size 5?
5) If you have the split/merge code written, why not allow splitting of engineering regiments into an HQ and 4 battalions? You could set it up so the HQ modifies the effectiveness of the battalions, e.g. (assuming they all cost the same) a battalion has 0.125*industrial capacity of a brigade, but any battalion in a formation with an HQ is has this doubled. So 4 battalions would give 0.5*brigade, but if they had an HQ this would be doubled to 1.0* brigade.
6) You could play the same game with ground combat ratings - being in a formation might act as a multiplier on combat ratings.
None of these are high priority for me, except 1 & 2 would be nice to reduce clutter on the ground units screen.
John