Tank first.
Stingray filters don't have very good media. They have a sponge, which is fine, but they also have carbon/zeolite cartridges, which are not. Do you have a manual for the filter? If you haven't, click
hereFish-in cycles are hard work. Are you prepared to do a water change every day? The easy way round it depends on your father. This will also fix the filter's shortcomings as well.
Does the main tank have sponges in the filter? If you would be allowed to, the simplest thing to do is cut some sponge from the main tank to the size and shape of the stingray's cartridges - and buy a new sponge to go in the main tank. Or take some ceramic media from the main tank if it has that, break open the cartridges, throw away what's inside and put the ceramic media inside instead.
Set the new tank up, get the water to the right temp, then sort out the media. Keep the mature media damp, don't let it dry out. Once it is in the new tank, move the endlers over. Test for ammonia and nitrite every day, and do a water change if you see a reading above zero for either of them. If you do get readings it shouldn't last long with mature media.
If you can't use any of the main tank's filter I would still replace the cartridges with new sponge. If the cartridges have been used by the last owner, they would need replacing anyway. But the best would be if your Dad would keep the endlers in his tank while you do a fishless cycle.
If that's not allowed and you have to do a fish-in cycle, you will need to test the tank every day for ammonia and nitrite. You will need to do a water change whenever the readings get near 0.25. If they go over that reading, it will harm the fish. They may not get sick right away but they will be weakened.
And it might mean doing a water change every day for a few weeks.
Fish.
The tank is fine for endlers and panda cories.
The main problem with mixing them is that pandas need cooler temperature than endlers. Panda's need 22 - 25
oC and endlers need 24 to 30. Keeping one at the top end and the other at the bottom end of their temp ranges is not good for the fish, they should really be in the middle of the range.
Pandas also prefer softer water than endlers.
If you still want to try them both, wait till the tank has been running a few months after it finishes cycling before getting the cories. Pandas are one of the more delicate species and it is better to wait until the biofilm, with all its micro-organisms, has established all over the tank tank and decor.
Panda cories need good water conditions. If you are not prepared to keep the tank clean, don't get them.