Adding fats generally softens the chocolate. You work out the % of fat based on weight.
For instance, if your formula has 10% full fat milk powder (assume 26% fat content), then the milk fat in your formula = 10% x 26% = 2.6% milk fat.
In terms of non-fats and tempering: I don't think non-fat ingredients (eg., skim milk powder) would affect tempering. I imagine it as a solid-type ingredient that just displaces sugar in your formula and gets ground up/refined with all of your other ingredients. This is just a hypothesis from my side, I've no testing or evidence to back it up.
If you want to sell your product and call it chocolate, there may well be limits to what additional fats and non-fats you can use and how much of them you can use.
In terms of other fats, I haven't used them. I would expect them to soften chocolate. I'm not sure how much, say, vegetable fat, you can add to normal chocolate before you turn it into compound chocolate. Most things I've read suggest 4-5% before you start affecting your ability to temper, but this probably differs based on the fat you are using. And as mentioned above, I think some fats can't be added to chocolate if you want to sell it as chocolate.
For some reason I have in my head that, as a rule of thumb, you should try and keep the additional milk fat from milk powder to <15% of total fat (ie., total fat = cocoa butter + additional fat) to minimise any problems with tempering. I'm sure that isn't a hard rule and depends on what type of milk powder you're adding, but it might give you a starting point for testing. So for a 40% fat chocolate, that would give an upper limit of 6% (40% x 15%) milk fat. If your milk powder was 26%, then the milk powder in your recipe would have an upper limit of 6%/26% = 23% milk powder by weight in the total formula.
When I have made milk chocolate, I have added ~5% milkfat but have taken it as high as 6.5% - it depends what you're after from your milk chocolate.
updated by @gap: 04/29/16 05:46:24AM