When To Add New Fish?

Provided your ammonia and nitrite results stay at zero for a week without you having to do any water changes to get them there, you can get more fish. But only as many fish equal to a third of the fish already there. Too many at one go will overwhelm the filter bacteria and your ammonia level, and later nitrite level, will go up. The bacteria can multiply fast enough to cope with that third. Which means I'm afraid, 2 danios' worth of fish for the next addition. As you build up the stock one third of the fish in there will get more and more.
Between additions of fish you should check for ammonia and nitrite daily, and do a water change if you see either or both above zero. Once both stay at zero for a week without needing water changes, you can get the next batch.

As for what to get, you need fish that have the same temperature requirements as danios - that is, assuming you have zebra danios. Their preferred temp range is 18 to 25 deg C, with around 22 to 23 deg being best. Tankmates should be fish that are happy at 23 deg. You could look at pearl danios or white cloud mountain minnows as fish needing the same temps - white clouds also come in a gold (or albino) form as well as grey.
I'll see if I can think of a centre piece fish that would do well at danios' preferred temps.

If you'd like bottom dwellers, how about our recent fish of the week, panda cories though I would be inclined to get those last as they do better in a mature tank.