Can a T8 fixture be used in a T12 lamp fixture?

No. The ballasts are not usually compatible. There are some dual use ballasts, but if it is an older T12 fixture you are likely out of luck.

Quite simply, no. = yes but dont do it base type at the ends of the lamps. T8 is smaller than T12, and they don't fit in the same 'tombstones'(sockets). You'll need to get a T8 lamp for your fixture....

no

mostly it's Mild Steel

Mercury.