So to combine the info here basically there is a BPM counter and you can adjust your BPM counter to set the BPM for the track. Once you've set the BPM, anything you stick into the slicer will automagically adjust the slice timing to the current BPM.
Additionally, if you prefer to stretch the loop in the playlist ala Acid style, FL will adjust the BPM by applying the stretch function you select in the samples panel. Default is resample on mine I'm using v 7.
Either way you do it when you adjust the BPM counter FL will resample, restretch or reslice the loop so it's in sync with your current BPM.
Things to consider are ensuring your loop samples are in a slicable format (4/4, 2/4, 2 bars of 4 stuff like that). When using voice or speech samples by selecting the 'speech' option in the sample menu, you ensure FL will fit your sample without changing the pitch (mostly).
Isnt there a simple time-stretching option like in Logic? Double click audio file, functions, Time & Pitch machine, enter original pitch and pitch you want to stretch it to and process! Done done and i mean done!