All synchronous FIFOs receive two clock inputs and they can be independent of one another (except tri-bus FIFOs that have 3 clock inputs). The -xx marking after the part number usually refers to the minimum clock cycle time (in ns). For example a -25 device has a minimum 25ns cycle time which is equivalent to a maximum clocking frequency of 40 MHz. You can change the frequency of the clocks as you wish. There is no set value. You're free to give the FIFO any clock rate, as long as it does not exceed 40MHz. So, if you were using a -35 device, you can replace that with a -25 FIFO. There is nothing to be added to the device if you switch speed grades. You can try it by tying the outputs of a clock generator to the device's read and write clock inputs. The FIFO should work at any frequency up to the maximum. The max. frequency for every synchronous FIFO is quoted in the data sheets. Certainly, if you have long clock traces on your PCB, which make the traces behave like transmission lines, then some capacitors and resistors will be required for line termination to maintain good signal integrity. If you are using two different clocks (one for each port), then you will need to find a part that will work for the faster of the two clocks. For example, if you have one port running at 100MHz and another at 67 MHz, you will need a FIFO that meets the 100 MHz clock rate.