You can use any frequency you desire - provided your circuit is designed for it.
With CMOS chips, frequency is related to power consumption. So a 25KHz clock would consume less power than a 32.768 KHz clock. 35 KHz clocking would comsume slightly more power. You should do the math to determine your proper minimum/ maximum clocking, coordinated with the actual chips you select.
There is a tradeoff between clock speed, power consumption, and the amount of work you can get done per clock cycle. This varies from circuit to circuit.
RTCs as a class, are most concerned with power consumption when the main power is off - and you are running on the back-up coin-cell battery, but also still need to be reasonably accurate clocks as well - within a few seconds per month typically