In my opinion, you've got to play the adventure before you get a good idea of the best answer to this question. Having done this myself, I feel qualified to give some advice to anyone coming to this module cold: don't have more than six players unless they like being bored. Simply put, combat is central to Operation Rimfire, and Mekton Zeta is not the quickest combat system around (for instance, damage from each shot in an autofired burst is calculated separately). When you add in the extra delays caused by players who don't plan their actions until it's their go and the GM having to process separate turns for all the bad guys, it just takes too long to get through a fight.
If you agree with my decision, the next thing to do is work out how you're going to put six players into ten characters. Some more advice: don't let them pick from the full list. Without going into details, some of the characters have much greater plot and story impacts than others. I think it would be a mistake as GM to allow the players to choose the less important characters over those that get to do more, decide more and interact more.
You may disagree, but I strongly recommend that you allow your six players to choose from a shortlist of six characters, the six of whom make up Fireflight Squadron. That's right, four of the characters in the module don't make it into the squadron - a major part of this is to reduce the number of participants in fights, so what good is it to keep them around as NPC squadron members? Don't worry, three of them get to stay around as reserve members stationed on the Rimfire. Only one of them should be dropped entirely, in my opinion, as hopelessly out of place and without worth.
So, who made the cut? Who got dropped? And why? Read on, and all shall be explained - without spoiling anything in Operation Rimfire, either.
First of all, we need officers. Fireflight Squadron is supposed to be an elite force, so it can have several officers despite being such a small unit. Thus, the first character to make the cut is Lieutenant Cedric Artis. Yes, mock me if you will for favouring my character above all others, but in my opinion the senior officer who's an experienced (and somewhat traumatised) combat veteran offers so many roleplaying and interaction opportunities that's he's a shoo-in. One.
Second is Lieutenant Laian. Anyone with an unusual background and scary mental powers is well worth having in a group of characters. This also gives Lt. Artis a peer to interact with if he doesn't want to slumming with the ensigns. Two.
The next two characters easily build off Artis, in my opinion. Captain Larani Kynis and Ensign Tammis Veric could be defined solely in relation to the lieutenant (Kynis, the stable officer; Veric, the young hotshot) and help bolster the international flavour of the Squadron. Three and four.
Any anime combat group needs female characters with small eyes and an iron heart. Ensign Karin Myiian meets both conditions admirably. She has skills no-one else has from her background in military intelligence and her personality type adds variety and tension to the Squadron. Five.
My last choice was the hardest. Personally, I find Ensign Tharis Tremaine annoyingly superior. His stats are above average across the board, his family were wealthy and important, he seems so ... toweringly noble. Bloody romantic leads. I really wanted to include Ensign Aleoi Hiarri in the squadron as the Ettaran representative, but she just isn't as good a combatant as Tremaine. If I was building a combat squad, would I take Mr Perfect or a teenaged computer whiz?
Feh. Doesn't mean I have to like it. Six.
After the last choice, it shouldn't be a surprise that the first Rimfire crew member with mekton piloting skills is Ensign Aleoi Hiarri. Fun-loving characters are good to have around, and she has very good skills in her area of speciality. One.
What's good for one Ettaran is good for the other. Ensign Eiran Alestain is an interesting counterpart for Aleoi: equally young, but shy where she is outgoing. He's also very good at what he does, and what he does is repair mektons. Two.
The last choice is the character I always thought should have been a Rimfire crew member when we were playing: Ensign Celine Attara. What's her speciality? Astronavigation. That's right, her specific talent is to help pilot spaceships. Why is Attara in the Squadron? Certainly not for her combat skills, let's put it that way. Three.
Ensign Daric Tallus. From a "GM as recruiter" point of view, what does he add, exactly? The six Squadron members are all good mekton combatants, with the exception of the telepath (and that makes up for a lot). The three Reserves have useful non-combat skills, and it's certainly a good idea to have back-up characters around for when a player's first character bites the big one. So, what does Tallus have going for him? He's really good with swords. And knives, too. Hold me back.
"Hopefully he will teach the others some of his skills, and in return he will learn from them." Yeah, right, General Haversom: don't you think it would be better to get your Squadron members to learn something with more general application, like Paramedic? And why have this guy on the team "to learn how to fight"? Ship him off somewhere else for some basic training, and leave us alone! Sheesh!
So, there you have it. A six-person Squadron, with three designated Reserve members in the Rimfire crew. Everyone's capable of holding their own in combat, they all have some niche-protection, the Fireflight Squadron seems more like an elite unit (50% officers!), and there are stand-by characters already around in case of accidents or noble sacrifices. In my opinion, this is a much stronger and more focussed gaming set-up than the default "ten players for ten characters" Squadron envisaged by Operation Rimfire.