That's great, amis! I should think (he's 12" long right?)that your feeding regimine noted should be just fine. As far as fuzzy mice are concerned, depending on his size ratio to theirs, offer one as soon as you think he's large enough to eat it without difficulty. There will be more food value such as calcium and vitamins as well in this "more mature" food item.
Mice are a whole food item, offering everything he needs including (I theorize only), roughage from the hair. This, since he isn't collecting much if any natural, coarse material that might cling to his food in the wild. Many people are now feeding mice only to their nerodia with no evidence of negative results. One potential of note is the likelyhood of obesity if overfeeding occurs so, caution should be used. Since nerodia are famous for being heavy feeders (due to lower energy "cold-blooded" food), their willingness to wolf down too much of anything is inherent in them.
I think you're wise to offer some variety such as trout, whole or peices. This will reduce the risk of obesity (if common sense prevails). Besides, despite the blanket statement above "everything he needs ..." I chose to err on the side of caution. Variety is the spice of life. Anyway, doctors tell us that eating fish once every few meals can reduce stroke and prostate cancer. Love your snake, feed him fish and steak! Sorry, mice didn't rhyme.
With a variety of food being offered, added vitamins are probably not necessary, however again, once every 2 - 3 months won't hurt as a precaution.
Michael