I went to use P-clips on an older bike and found they just didn't fit. This was on a multispeed bike, though, not a single speed. When in high gear in the back, the chain rubbed against the P-clip. As long as you have enough clearance there (which with a single speed I would assume you do), you should be okay.
Load carrying capability is not a problem. I have baskets attached with P-clips on my Raleigh and have loaded the bejeebus out of it (gallons of milke and other groceries) with no problems.
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I stop for people / whose right of way I honor / but not for no one.
Originally Posted by
bragi 
"However, it's never a good idea to overgeneralize."