Moar brain dump. When you connect PV panels in parallel you can get away without fusing two strings. because even if one string of panels shorts out, they are engineered to accept twice as much current as they produce.

Check the label on any panel and you'll find numbers like Isc 13amps , fuse at 30amps. (Isc is amps short circuit, the max amps the panel can ever make).

However when you combine 3 or more strings of panels now you have a situation where one string could malfunction and the two neighbors could dump amps into it. So when you parallel 3 or more sets of panels you must fuse each individual string at the manufacturers spec (ie probably around 30amps).

There are inline fuse holders that have the MC4 connectors or you can use a combiner box with fuses or breakers.

One more tip... do NOT disconnect MC4 connectors while amps are flowing. Use a shutoff switch or power down the MPPT or inverter before taking MC4s apart. If you take them apart under load they will arc. The arc will destroy the contact surface and about 6 months from now your connector will mysteriously melt.