Highlighting stats is an out of character choice, player to player. I think your crew needs to sit down and talk about this amongst each other, and by talk, I mean seriously talk about why certain choices are being made. If the player is highlighting the gun-luggers cool, and he doesn't like that, instead of crying about wanting more, maybe he should ask, "hey buddy, why cool?". There is a certain level of expectation that the players actually like each other and want to tell an awesome story together. This is directly opposed to the idea that every choice will always be optimized to help them level up faster.
As for the stats themselves, you have a variety of choices. One of the simpliest that I used for about half of the games I ran, was for for my group, you got experiment based on the stat that should be rolled, rather then the one they've substituted it for. This was the favorite choice out of my group.
Others on these forums have suggested that making a HARD substitution means something different then rolling the move based of the cool stat. As in there is a distinct flavor that cool has that hard does not, and that playing it off as cool and not hard would earn the point, but bulling through with the best stat therefore wouldn't get the same treatment. I am not a fan of this personally, but it is an option.
The most straight forward is, if it is that stat then great. If they have no moves involving that stat, they might take that as an act of aggression. This is also fine. I've seen a lot of pvp in my games, and these types of confrontations have been used before to signal that someone actively doesn't want the other to have as many options available to them. Perhaps slowing them down? That is perfectly reasonable.
However OOC group cohesion is essential. You must sort this out amongst your own crew, and they must set aside bulling, threats, or hostility amongst each other in order to construct a fun game. The only real answer here is, you need to find out what the answer is by talking to them and making everyone get on the same page. As an MC I've chosen stats of players that had a -2 before. Firstly, I've told them it's so I can see if they can pull it off. It's a dare. Secondly, because if they avoid making any type of roll they aren't great in, it's not really a character weakness. Weaknesses should be celebrated within the story just as much as the strengths matter. There have been times when the stat is simply 100% ignored by the player no matter what, and that's fine too. But highlighting it isn't being mean, it's expanding the current patterns, even if that means they might not progress if they just want to kick ass all the time instead.
As for the Hard sub for act under fire, maybe it means exactly what it says it does. "I want to see you do cool things outside of the combat" That might mean sneaking into a place rather then gunning everyone down. It might mean jumping from one moving vehicle to another without threats of death. It might mean running out in front of some type of bomb to grab a kid and throw them and you into safety. Maybe it means lying to someone when getting caught in that lie might be very bad. It means, stop fucking shooting everyone.
Good luck.