[Peter Finke]

Bartian, the smallest linkei that I saw breeding were about 3 to 3,5 cm (the male). But the length is not the decisive factor. The fish must be in breeding condition, and that depends mostly on their food. Freshly hatched Artemia naupliae are good, Moina are good, Grindal equally, Cyclops or Culex are very good and white glassworms are best. Don’t underestimate that! Especially young females must get in that condition first, and that takes time. And you can judge that mostly from behaviour.
It is very advisable to keep even young fish under best circumstances in a tank with the right water and with caves. Then in principle breeding is possible there, too. In any case, you should observe the behaviour very closely. If the males begin displaying, the females are often not yet ready. They must respond by changing their colours and getting lighter, and react to the males gestures. Then you should observe the first attempts of the male to attract the female to a cave an into the cave. All this should happen in the normal tank, and only then, when that happens, the time has come to catch that pair and transfer it to a small breeding tank (if the normal tank is not conveniant for that purpose for whatever reason).
I do not distinguish between breeding tanks and tanks simply for “keeping”, but I admit that this is sometimes reasonable (for instance if you have a lot of young to raise).