A facia board is not required, you can nail directly to the rafter tails. There are at least 3 different gutter hanger hardware sets. One, the spike and ferrule, problem is the spike is aluminum too and bends easy; two the wrap around hanger that has a strap going up onto the shingles to be nailed down with 2 or 3 roofing nails, to me its unsightly; three there is a clamp on the top of the gutter hidden from sight with a short nailer at the rear, so you can use one nail per rafter tail, you just need a 'pea shooter' nailer. That is, a piece of tubing large enough for the nail head to go through and a rod that is a little longer than the tubing. You put the nail in the business end, place that end where you want the nail, strike the other end of the rod to drive the nail.
The aluminum should not be touching the tin roof and there will be no problem with electrolisis.