The best piece of advice for a fish-in cycle is to take some media from the tank already in the house. For 3 endlers in a 25 litre tank you won't need much, but it is safe to take up to a third of the mature media, monitoring the ammonia and nitrite readings in that tank for a few days to be on the safe side.
Once you have the new tank, fill it with dechlorinated water, and if you don't use warm water wait until it has reached the right temp with the filter turned on to move the water round but no media inside the filter. Then when you are ready, take some mature media from the main tank's filter - don't clean it - and put it into the new tank's filter. As soon as you've done that, put the endlers in the tank as well.
Feed the fish less food than normal for several days as less food = less ammonia, and test the new tank every day for ammonia and nitrite. If you see a reading other than zero for either of them, do a water change. You need to stop both of them reaching 0.25 by doing water changes. When you've tested every day and had zero readings for 7 days in a row, then the tank will be cycled.
The gap left in the main tank's filter will need filling. Either put the media that comes with the new tank into the main tank's filter or get some new media for the main tank.
If the endlers breed while you have readings above zero, they will add the the amount of ammonia being made. Let this batch get eaten if that's what the adults decide to do. You will have more soon with 2 females. Up to 20 fry twice a month. And don't forget that the male endler can impregnate the female fry when they are as young as 3 weeks. They can't have fry that young but they can store sperm from that young. And as soon as the male fry reach puberty, they'll also be able to mate with the adults females and female fry. Then you'll have up to 20 fry from every female every month. Unless you do something, there won't be room for the fish to move. And in my experience, even if a shop will take fry, they won't take them until they reach the size of the ones you've just bought.
I used to have pure endlers which I got from specialist dealers. Because they were pure not hybrids like yours will be, I did sell them quite easily on aquarist classifieds, then once everyone in the area had them I sold them to shops. I didn't get cash for them, only store credit. And they wouldn't take them till they were fully grown.