I believe Caddy built both the M5 version of the Stewart and the M24 "Chaffee" light tanks. Both were recon vehicles. The Stewart had only a 37mm gun, which was increasingly lacking as the war went on. The Chaffee had a 75mm gun which gave it as much firepower as the standard M4 Sherman, but had less armor and relied more on speed and manouver rather than try to fight toe-to-toe with anyone (as with most recon vehicles).
The M18 Hellcat was considered a tank destroyer. It was fast, had a decent gun (76mm), but very lightly armored. I googled it and the first sight says it was manufactured by Buick. It also says maximum hull thickness was a half inch and max turret armor was one inch. By comparison, IIRC the M4 Sherman had 2 inches armor in the front hull.