Hah, yeah, Elethea uses two daggers, it's the fastest combination there is. I think you do need some strength, though, to be able to punch through the weak spots of someone wearing full plate. But, yes, their body type would be closest to that of an acrobat, counting on finesse and agility rather than brute force, wearing the lightest of armor. I think such people could probably run rings around plate wearers - they'd have to, to be able to penetrate fine armor.

I think all plate wearers will tend to be muscular, because even if one can wear forty-five pounds of plate armor (the number given on Wikipedia for a suit of well-tempered steel), you need to be able to fight for a considerably long time wearing it. Not to mention marching in it, or traveling any distance in it. Endurance is god for any soldier, it doesn't matter if they're shield users or not.

As for crossbows, I think it depends on what the cocking mechanism is. If using stirrups, there needs to be some strength involved, but not if using a cranequin. Crossbowmen also carried pavises, so they couldn't be weaklings, either.

According to the study of the possible longbowmen skeletons on the Mary Rose, their left arms were considerably thicker, had bone spurs, and there was a special bone separated from their shoulder.