Okay guys, here's how it works:
You get one catch for all Toughness powers except Physical Immunity, it can't reduce their total cost below -1, so if you have Inhuman Toughness only with a Catch of Cold Iron, Cold Iron's only worth +1 (and you could grab two more points of powers, effectively for free). Feel free re-read where this is laid out in detail on YS p. 185.
Physical Immunity gets an entirely separate Catch referred to as the Stacked Catch, which has it's own rules listed on YS p. 187 This is listed under Physical Immunity and is an explicit exception to only having one Catch, that applies only to Physical Immunity.