Fan is a fan. They all report rpm the same, revolutions per minute. The software / motherboard / pump doesn't care what the actual rpm is, it only concerns itself with what it's doing.
So you'll have s fan curve that's set for temp and speed. The speed is represented as a % and is gradient from 0-100%. Almost all default curves set 70°C as 100% and the curve goes down from there.
So you can use a 200mm case mounted fan on a 120mm slotted radiator mounted internally. The only issue being loss of efficiency. 200mm fans aren't especially high static pressure, they are built to move a lot of air, not move air a lot. So you'll get minimal air flowing through the rad. A 120mm fan mounted directly to a 120mm slotted rad has little seepage, it's pretty well shrouded to channel its output directly into the fin array. A 200mm Fan on a 120mm slotted rad has tons of seepage, so any turbulence, any restrictive force applied to the air on the fin array will just divert the airflow around the rad instead of through it.
It'll fit, it'll work, it just won't work very well at all. You'll get much better efficiency and much better temp control with the 120mm fans.
Just use fan extensions. Link the fans to the cooler system instead of through system-fan motherboard header. Any fan header, be it on the pump or motherboard is the same 12v, so the fan could care less about source, it's the source that cares about the fan reporting back to it.