I would arm them as follows...
Chris (Yul Brynner): Colt SAA Revolver, Winchester Model 1892 Rifle
Chico (Horst Buchholz): Colt SAA Revolver, Winchester Model 1892 Rifle
Harry Luck (Brad Dexter): Colt SAA Revolver, Winchester Model 1892 Rifle
Vin (Steve McQueen): Colt SAA Revolver 5.5", Colt SAA Revolver 7.5", Double-Barrel 12-gauge Shotgun
Bernardo O'Reilly (Charles Bronson): Colt SAA Revolver, Winchester Model 1892 Rifle
Britt (James Coburn): Colt SAA Revolver, Knives (Throwing)
Lee (Robert Vaughn): Colt SAA Revolver
You're right that they may be a little flat and redundant with the core IHMN rules. Dedicated Old West rules might provide more texture to conventional heroic types like this, but I think you could get a nice IHMN company out of it. With two more Talents and a slight variant of the SV stat, you may be able to create a fun group to play. Gunslinger does not seem the correct Talent as none used pistols in tandem.
Allow different SV for weapons. Bernardo should be excellent with the rifle, but only good with the revolver. Chris and Chico should be excellent with the revolver, but only good with the rifle. Britt should be a knife-throwing, fighting(?), specialist.
"Fast Draw" After the first figure of the allied company activates in the Shooting Phase, roll a d10 for each figure with 'Fast Draw." On a 9+, they are allowed to make a free shooting attack. The figure may shoot again in the same phase when activated.
"Lack of Confidence" Until this figure kills an opponent worth at least 50% of his point cost, he is hesitant to act in both the Movement Phase and the Shooting Phase. Roll a d10. On a 1 or 2, he hesitates and fails to act. Additionally, he suffers a +2 penalty on Pluck tests against Terrifying opponents until he regains confidence.
Lee should get 'Lack of Confidence' and 'Fast Draw' and a Speed of +2.
Chico and Chris should get 'Fast Draw' and a Speed of +2. I would give Chico a modest stat line for SV and FV.
Vin, Bernadro and Britt perhaps should be Tough or have high FV.
Chris should have Leadership.
I would rate Lee the weakest (lowest points) followed by Harry and Chico.
Chris seems the most powerful followed closely by Vin and Bernardo.
For equipment options...
http://www.imfdb.org/wiki/Magnificent_Seven,_The