My headcanons about the body types and fashion styles of the Hellions!
Roulette: Petite, curvaceous, pinup figure. She's officially listed as 5'1 and 90 lbs but I don't think the latter can be correct, she's gonna weigh more with a body like this, even at such a compact height
http://images2.wikia.nocookie.net/__cb20050408031560/marveldatabase/images/9/97/Roulette_001.gif
http://img2.wikia.nocookie.net/__cb20100719162326/marveldatabase/images/7/7f/Hellions_(Earth-616).jpg
As a note, I've noticed that the stats-makers consistently seem to underestimate what female characters reasonably should weigh given the way they're drawn, so I'm not surprised by this. Anyway, Jenny's probably older than her years due to her rough start in life, and her body reflects this, as the most 'developed' and 'womanly' of the Hellions girls. I think she likes this, and prefers to think that she's a sexy adult because of it, when, really, she's not, she's just a kid who had to grow up too fast but is still far from the grownup she thinks she is. You see this in how she dresses too; I picture her going for very "sexy 80s badgirl" looks, like leather jackets, leather miniskirts, etc., trying to look simultaneously glamorous/classy and like a tough punk, reflecting both the elite Hellfire Club and her gang origins. Definitely wears a lot of black and red. Probably piles on as much jewelry as she can because she wants to show she's a stylish lady living the high life now but she doesn't actually know how to pull it off and she just ends up looking like a trashy overdone mess but she doesn't realize it. She tries too hard a lot, I think.
Tarot: Slender, willowy, weak-looking and probably the most physically frail of the bunch. She's listed as being 5'7 and 115 lbs and that actually seems about right. I imagine her having a long thin body and long thin limbs.
http://img3.wikia.nocookie.net/__cb20140316000204/villains/images/5/5a/Marie-Ange_Colbert_(Earth-616)_from_X-Force_Vol_1_87.jpg
We actually do see her in civilian garb once, and it's this adorable little sailor girl getup. This was when Louise Simonson had taken over writing the New Mutants and Tarot had started to really come out of her shell, as well as losing her overt French accent, so going off that, I think that in her earlier appearances, when she was shyer, she would have been the type to hide in her clothes. She'd wear long skirts and baggy tops and thick tights, all in dark drab neutral colors, but never anything as stark as black because that would make her stick out. Once she starts getting more confident, that's when she starts going for a cuter, brighter, prettier look with a much more defined style, a sort of "innocent retro European schoolgirl gamine" type of wardrobe. I'm told that she wore a more Gothic look when King Bedlam temporarily resurrected her, but I don't have those issues and haven't seen any scans. If I were to bring her back again, I think I'd have her trying to hide in dark colors and big clothes like I imagine her originally doing, and then gradually moving towards a more confident look that has some Gothic/witchy influence, but also some of the cutesy and pretty of that sailor dress look as well.
Catseye: She's listed as six feet and 120 lbs, and that's really underweight for that height, but unlike with Roulette I think that's intentional, because she was probably super-skinny when Emma found her, and her first appearance is drawn rail-thin as well
http://img1.wikia.nocookie.net/__cb20061124085755/marveldatabase/images/2/26/Sharon_Smith_(Earth-616)_0002.jpg
But then later she fills out into more average (for comics) frame
http://static.comicvine.com/uploads/original/4/49448/3521278-catseye.jpg
I like to think she kind of ended up the Amazon of the team, with everything she eats getting metabolized into lean, hard muscle. I think it'd make a good contrast to sexpot Roulette and skinny Tarot. As far as how she'd dress...basically, whatever they could get her into. We do see her in civilian clothing a few times: the fancy yellow dress she wore to a Hellfire Club ball, a fuzzy orange sweater with a matching long skirt and flats, and this little pink/purple ensemble that, to my shock, seems to include heels?? I'm really surprised they were able to wrangle her into some of this stuff! Of course, this was all after she'd been on the team awhile, so there's that. But in the very beginning, I think the most she'd stay in is stuff that's very comfortable and not at all restricting in any way, probably things either baggy or skimpy or that are skintight in such a way that, like her Hellions costume, it actually allows her greater freedom of movement than normal clothing. I don't think Catseye cares about color, but I think Emma would notice that she looks good in orange, yellow, and pink/purple, which is probably why those are the colors we've seen her in. Likely hates shoes too.
As far as the boys go, I don't have much to say about their bodies. Thunderbird and Beef are obviously big hulks, and as for the other three, I do picture them as having differences in their bodies, but in such a way I think I could only articulate it by sketching them because it's not as overt as with the girls---they're more like three variations on one body type than three separate types. So, I'll just talk about clothes with them. Warning, I have no idea what I'm talking about when it comes to how guys dress.
Thunderbird: IDK like typical guy clothes?? I know he goes on to be a big player in X-Force and stuff after his time with the Hellions so we probably see him in civilian clothes there. If I had to guess I'd just say he probably dresses pretty practically and plainly, but not in a way that suggests he doesn't take pride in himself.
Jetstream: Casual athletic to casual nice. Gradually grows from being sure to hide his prosthetic legs in public to having no shame in wearing shorts outside.
Empath: We see him in stuff like blazers over T-shirts and sleeveless sweaters over long-sleeved shirts so I guess preppy, which fits his snobby attitude
Bevatron: I see him as cutesy and kind of effeminate, so tight jeans and tight shirts with fun logos, okay with showing a little midriff, always looks put together but not like it's on purpose.
Beef: Anything that fits? Jerseys and tank tops and stuff. Tough looking, macho, sporty, military-surplus junk.