controlling tempo of an sf play object?

First off, take a look at the [transport] object, see if that makes you happier than tempo.

Second, as far as play is concerned anything like that is gonna take bang as input. as long as you get bangs out of the step sequencer interface in the order and rate you want, playing the files will be an afterthought. Take a look at the msp tutorials for the rest of what you need.