We would recommend following the kit booklet guidelines on the centrifuge speed. 

However, you can centrifuge the sample at the lower speed for longer if you do not have the equipment for the higher speed.
The most important thing for your sample is for a pellet to form and the supernatant to be clear following centrifugation.

 
If this is not possible with the lower speed, we recommend using filtering your sample using filter paper prior to the next step in the analysis. 

We also recommend if altering the centrifugation speeds that this is validated using the provided control. If the provided control gives the correct value as stated, you can assume the changes to the centrifugation speed have been sufficiently adjusted.