mice for life is a good plan, when adult just get an adult mouse and if your GTP gets that big that an adult mouse seems way to small get extra big mice

i would niot recommend feeding mutiple prey items in one feeding..
i myself have fed them both mice and rats, the one i have now is feeding on rats..
nothing wrong with rats, but they are more nutritious and can fatten up a GTP if fed to often !!
remember the don't feed to much rule

i feed mine rats but feed smaller rats than what i would a mouse for her size and at longer intervales..
these are slow growing snakes, nothing like other snakes that you see a lump after the meal..
rats have bigger bones and thicker fur, some will say this is cause for prolapses.
I have never had a problem with that, but then i feed smaller prey, but it could verry well happen, so care is a must..
in fact my arboreal years have made me a strong believer of feeding smaller more often then big less often.
i do this with all my snakes. even my adult ball pythons only get a large rat once a week
they grow nice and slow versus getting fat fast.. last thing you want is a fat GTP
best thing is to go with what you have available.
if you can stick with mice, do so
if you need bigger, rats are good but feed smaller
soft furs are great , don't know about the area you are , but in general, they are more expensive, and are harder to find.. plus many snakes will refuse anytrhing else after soft furs, so if they are hard to find !!! you may create yourself a problem you'd be best avoiding..