Is it possible to create a personal ward that only selectively blocks things?
Example: Master of Metal wants to be warded from metal weapons, but not a needle used to stitch wounds, or a razor to shave with. Is this possible? An intellego prerequisite or something?
Also: in a prior adventure, Master of Metals wanted to 'ward' an old knight so that he could not put on armor or wield a sword, thus forcing him to not take part in a tournament that would surely have killed the old man. Would some variant of a ward be the right approach?
The rules neither specifically allow nor prohibit such, so I think that if it's kosher with your Troupe, then, yes, you're on the right approach on both counts.
It's clear that you can be somewhat selective about what Forms of Terram are warded against by making use of the magnitudes - the lowest ward blocks only "dirt, sand, mud or clay", the next would block that plus stone and glass, and the last would include metal (and gems) as well.
The key phrase would be the last in that first paragraph in the ReTe Guidelines - "Manipulating objects with great precision may also be higher, depending on the level of precision". If you include warding as "manipulating" things (in the general, not specific sense of the word), then it's clearly possible, for either mentioned effect - and "you" are the one who makes those decisions.