Hello F-22
I have the AIO in use for a 9700k and use it in push/pull with the supplied fans and the Corsair 120mm RGB fans on the visible front, works great. So yes, you can install any 120mm fans. On the temperature question: My 9700k runs at a maximum of 55° C under full load after 20 minutes of overclocking stress test... In my opinion, you still have enough reserves with the 9900k ;)

How much would this affect the cooling performance and is this aio suitable for cooling an i9900 k that may be slightly overclocked?