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)...
