I second the idea that the hats should be a different colour, but I don't think they should be grey - go with the same colour as the jacket (so, dark green). I would still actively encourage dremmling the top "bulge" off them, so that the sides are flat - both so that they don't look like every other mordian, and so that they will look more like the hats you showed us. If you have a power tool, this step would actually be quite easy.
BTW, the tall caps would look better, but would still require you to trim down the hats (otherwise you would have a big bulge half way up the hat) and, as you would also have to green stuff them, would be more difficult to do than the shot caps.
Oh, another way to get rid of the top bulge would be to can the top of the cap off with a jewel saw or a dremel. This might be more fiddly (I would have to try both to know for sure), and you definitely would have to green stuff the cap up to the desired height.
I think the taller plume is a good idea. Drill a small hole on the top of the hat with a pin vise, then glue in a short bit of pin or paper clip (I think pin would be better, so the plume doesn't get too wide) of the desired length. Then wrap this bit of pin in green stuff and detail it with a pin to make it look more feathery. Should look good - though, if you don't dremmel the caps, it would look pretty silly, IMHO.
If you go for white trim on the uniform, I would also go for white trim on the pants (I still think yellow would look better, but they are your models, so you decide
). I wouldn't
green stuff the strip though - it would be much to easy to make the strip either irregular or too thick, and I don't think those risks (and extra effort) are worth it when almost the same effect can be acchieved with paint. Just paint a black stip down first, and then the strip of white or yellow covering the black. If a tiny line of black shows on either side, great, so long as it never gets too thick.
I don't think black lasgun would look too futuristic - but another option would be to paint the casing like wood grain - that might look cool.
Inquisitor Psychologis Ruminahui