With SolarEdge you have series strings of Optimizers, not series strings of PV modules.
The SE9KUS has two DC inputs on a terminal strip (the two input strings are parallel connected at the input terminal), each terminal position is usually used for a series string of Optimizers. The datasheet shows 'Maximum Input Voltage DC+ to DC-' is 500 VDC and 'Maximum Input Current' is 26.5A. Maximum DC Power (Module STC) is shown as 12,150W.
The datasheet for the P320 Optimizer shows that for THREE PHASE 208V 'Minimum String Length (Power Optimizers)' is 10, 'Maximum String Length' is 25, and 'Maximum Power per String' is 6000W. Maximum Output Current is 15A.
The original post does not identify the PV module rated power, but if it is 300W or less there would be no problem connecting one string of 20 and one string of 19 Optimizers to the inverter. No fuses required. If the PV modules are over 300W each, there is a problem.
The discussion at a SolarEdge training session was that the inverter will adjust the outputs of the Optimizers such that the inverter input current, 26.5A in this case, is not exceeded.
The installation instructions do state "You can connect systems with multiple DC strings in parallel to the DC input terminals of the switch. Inverters have one, two or three pairs of DC input terminals, depending on the inverter power rating. If more strings are required, they can be connected in parallel using an external combiner box before connecting to the switch." These are strings of Optimizers and the rules based on PV module ISC and related fuse requirements do not apply, but the output current of the Optimizers must be considered for wire sizing. If fuses are used to reduce wire gauge for each string, they are based on the continuous 15A of the Optimizer, 20A in this case and the 20A is used for calculation of the required wire gauge as generally done.