No, they can reach 12 -15 inches. They absolutely don't stay small. And they need companions just like corys do, 4 to 5 others. You have to do your own research most of the time. The pet store guys don't always tell you what's right. They tell you what they need to in order to sell a fish. 6 ft tanks are recommended for them. I would see if you can return the loach when you get the other fish and maybe you can get credit? You'd save yourself some money that way.
BettaMiah the cories would be okay in a 29 gallon tank. C. aeneus don't get larger than 3 inches and even thats a bit of an overestimate. C. julii don't grow more than 2.5 inches. Many websites say that you can keep a small school of either species in a 10 gallon tank (ie 4 fish). I have four C. aeneus in my ten gallon and they are all happy and healthy and prove it by spawning quite regularly. Eight in a 29 gallon should be fine provided they have enough swimming space on the bottom and the tank has enough filtration to keep the water healthy and oxygenated with the other fish.
OP, please don't return any fish, but work towards giving everyone their needs the best you can. I know you have girls payed for, is there anyways your parents would let you get a second hand 10g for them? Then you could get better groups in your 29g! But if you can't, work with what you have and find a way to get a second tank for your girls. Craigslist and local tropical clubs is a good start. Like you, I was given three types of fish when I started tanks and had to improvise. My guys are doing pretty good with their set ups. Mind you I had to set up a 10g to remove my Glowlight Tetras since the main tank scared them, now I have a gentle dwarf gourami in there with them. Work with your parents on a second tank, it will help you tons.
The second tank is not an option. The money isn't an issue. My parents jsut don't want anymore big tanks in my rooms and the only place they will let me put them in the basemeant which is under construction due to water damage.
My parents also refuse to get anything second hand or used.
So the only thing i can do is keep them in small groups for now.
I could have gotten a free 10 and a free 20L with stands on freecycle but my parents wouldn't let me.
Well your room will be done in January, upgrade or get another tank then!? I understand parents blocking the tank issue, my Mom would be the same way lol I have THREE danios that do well together in my 29g, every danio I bought they killed. I had to upgrade the 5g they came with to the 29g. Tis the life of fish, you gotta change things up to meet their needs. Hope you can find a good balance in your 29 :)
My danios, which are the same fish, are not fin nippers but they are territorial. They claim the top of the tank as theirs. They are the sharks of my tank. Higher numbers may slow down boredom but they are still aggressive. You can try it, but watching my own tanks I would never put color fins with mine.