Does the Repair Drones ability of the Hoshiko Robotics Cruiser even stack?

nope. no stack.

General or Strategy forums would have been a good place, since this isn't just limited to Diplomacy.

Its supposed to be like that. The Vasari overseer has the only healing ability that does stack with itself, and that's because it heals instantly (though not as much as the Hosho). So the game is actually doing you a favor by preventing you from wasting your the other's ability when they could be healing your other ships.

Well, in fact I am pretty sure this is just because when you select a group of units, and order an ability from the action grid, you only order the sub-selected ship of the group to use this ability. If you order a move, they all move, but if you order an ability, only the sub-selected ship does it. So this is why only 1 hoshiko uses its ability.

You can see this behaviour on many other ships, and also by the fact that when you select a group containing multiple types of units, the abilities you see on the action grid are the ones of the sub-selected in the group. Another example is with the Arcova's Timed Explosives : if you turn autocast off, select a group of 10 Arcovas, click on Timed Explosives then on a structure, only one will go use it. If you want them all to use it in this case, you have to tell each one to do so : you can use TAB to sub-select the next unit in the group, press 'z' I think to use TE, click on the target, and repeat.

Aside of that, like Pbhead and GoaFan said, Repair Drones doesn't stack with itself, so you can't be repairing more than 20 hull/s with only hoshikos. If you explicitly tell a hoshiko to use repair on a target already getting repaired, you're wasting abilities.

You can stack it with repair bays and the the starbase repair however, this way you can get a nice 75/s total (20+40+15)...

@Pbhead, GoaFan77, in-the-sun

Thanks for confirming I didn't hit my head.

@in-the-sun

Thanks for the in-depth answer and proposed solution. That's exactly what I wanted to do (if only they didn't make it such a chore though).

@in-the-sun

Does this "work"/follow for ships like Subjugators/Subverters? I'd love to have only ONE Subverter disable a group of frigates/synergy turrets rather than have multiple ones stupidly autocasting over already disabled ships (unless it extends the duration, which I highly doubt due to potential abuse).

Yeah absolutely this is the general behaviour of the game, so for example to use subverters' distortion field effectively, select your group of subverters, bind them to a selection group, disable autocast. When the fight comes, you can select the subverters rapidly with the key you chose, press z to use distortion field with the first stilakus, click on your target, press TAB so you sub-select the next stilakus, press z again and click on another target, repeat until you have used all the subs of the group.

I don't know if there are other tricks like that that can ease ordering units to do what you want. The above is what I try to use with many ship types. Maybe some vets can help us with their own tips...