Your tank is large enough for more Yoyos, but it is better not to get just one or two more, sometimes they may squabble. Half a dozen is better, but your tank is barely large enough to do that.
IME they ate small fish. My Yoyos were about as big as yours (3-4") and they ate Neon Tetras that were just about full grown.
They will eat shrimp; most Loaches will. They are naturally predatory toward critters that live in or near the bottom such as snails and crustaceans. Like most fish, if it fits in their mouth (like a smaller fish) they are quite willing to eat it.
Never combine fish that are such different sizes that the smaller could be eaten. Usually this means that the smaller should be at least 1/4 the length of the larger, and 1/2 the length is safer. So, 1" Neons might be eaten by 4" Yoyo Loaches (I have seen it happen) but 2" long fish ought to be safe, but when the Yoyos grow...
A better Loach for your small fish set up would be the Zebra Loach
Botia striata
http://www.loaches.com/species-index/botia-striata
or the Dwarf Loach,
Yasuhikotakia sidthimunki
http://www.loaches.com/species-index/ya ... idthimunki
Either of these can eat shrimp, (Zebras ate my Cherry Shrimp). Maybe sids would not eat the adults? Sids did not eat adult pond snails.
Keep either of these in a group of at least half a dozen, and more (especially the Dwarf Loach) would be better.
The other alternative is to get rid of all your small fish, perhaps get a group of some species of Rainbow, or larger Tetra such as Congos, or one of the mid sized Barb. Do not get too many. With half a dozen Yoyos and a small group of something else large enough not to get eaten the tank is getting really close to the maximum bioload.
38 tanks, 2 ponds over 4000 liters of water to keep clean and fresh.
Happy fish keeping!
