I have heard several fruit beer podcast and they all come to the same conclusion, 1/3 - 1/4 in the primary and the other portion in the secondary (after fermentation). but I was wondering what would happen if I made a somewhat peach cider then blended it with a porter? am I in for trouble or do I have a good thing ahead?
You are basically veering towards the land of a Black Velvet made with cider (1/2 Guinness, 1/2 Cider) http://en.wikipedia.org/wiki/Black_and_Tan, so I don't think trouble is necessarily on it's way. I have done this with stouts and porters in the past and found them to be enjoyable. Instead of brewing it all together...make a really good peach cider and a really good porter. Once complete and kegged/bottled, mix together in a glass just as you would a Black Velvet. This way you can modify the ratio between the two as you see fit, or if it tastes terrible, never mix them together ever again. Plus, 2 batches is always better than one. ;)
You'll definitely make beer if you follow the process above, so in that sense you're definitely not in for trouble. However, I'd be careful mixing "peach" and "porter". The flavors don't sound natural together, but I've been surprised before.
You should test it out before you commit the whole batch. Try taking a small sample of your beer after fermentation has slowed. Mix this with a small sample of the peach cider, and see if you like it. If you do, you have a good thing ahead.
Also, consider adding all the fruit to the secondary. I did this with my only fruit beer (a Strawberry Wheat), and it turned out fantastic.