Never heard "rooster" for penis in the US or Canada. Where did you find it used? May be local.

Smalls, in Canadian-US auction terminology, applies to small goods such as chinaware and ornaments, as opposed to furniture, etc. Also heard used by store stockists.

Haven't heard smalls applied to male underwear (which does include undershirts in store terminology (US and Canada).
Of course many men only wear "briefs" (frequent term for underpants) and do not wear the undershirts (locally called "vests," dunno where that comes from, may be southern, since my wife from Georgia says they use it down theuh).