Sorry for jumping in a little late (travel, busy work on return, and no mind due to jetlag).
Anyway, there are three factors that can affect the allocation of MG, MTR, etc. in AI units. You found one. Yes, an AI Role Score = 0 will definitely do what you are seeing.
1. AI Role Score. Note that in vanilla, machineguns, mortarts, and bazooka all have AI Role Score = 0. You will have to pick if they are support or infantry. The amount the put into a unit is related to size of the score relative to other units with the same score. So, if they all have 100, then they will be about equal. For this, note that infantry guns and AT guns have AI Role Score = 100. So, if they are all 100, they will be roughly equal in the units. Historically, MGs should be about 3x MTR, which is about equal for Inf Guns and AT guns. So, adjusting the AI Role Score will help.
2. Rulevar 156 in the Artificial Intelligence slot. This determines the ratio of infantry to support elements. It is set to 60. So, the AI should make units that are about 60% Rifle/SMG with 40% support.
3. I think the production cost might enter as well. There might be a bit of a check on value, that is firepower per production cost when comparing units with the same AI Role Score. But, it might just be the AI Role Score and it fills them with a ratio specified by the AI Role Score. But, there might be an algorithm that compares what the unit gets for the cost.
As a start set Rulevar 156 to what you wnat, and then balance the AI Role Score. For example, 100 for MG and 33 for MTR, Inf gun and AT gun, will probably get a unit with equal numbers of MTR, Inf Gun, and AT gun, but 3x as many MG.
I'll have to go back and check just what I did.
Note that Rulevar 157 determines the ratio of armor in an armored unit.
As for NATO symbols, I am the polar opposite to you. I dislike the silhouettes, so I have exclusively used NATO symbols. This is especially true for the TOE system. It could be changed. For Four Seasons, I would have produce a set of silhouette images to overwrite the NATO symbols I use. It is way to complicated to have a separate version that uses them instead. As for the TOE, in principle, an additional set of silhouette images could be added (although, I have a lot symbols there).