Well, a few factors to consider on tankmates...
1- The Glass Cats are said to need a higher pH, of 7 or greater, if I read correctly. So, you would have to read through some profiles, or get input from members who know pH needs of fish off the top of their heads.
2- Their temp is in the 70s so, any of the higher temp fish wouldnt work, which is okay since I have noticed a lot of higher temp fish, such as rams, need lower pH.
3- They need calm tankmates and limited current... in that department it would be fish like rasbora and maybe cherry barb, and some of the calmer nicer tetra, such as neon tetra.
4- They like soft to medium hard water, so livebearers are completely out due to this as well as them not being calm.
5- You could get away with them in the 29g, IMO, others may suggest longer due to the length needed, as they tend to be long. So, other fish would need to be smaller for sure, and limited in numbers, IMO.
I think Cherry barb are a nice fit, they do best though in larger groups though. But water parameter wise, they would work, and they are pretty calm fish.
Harlequin Rasbora would work if the pH is close to 7, but not higher.
Zebra Danio also work regarding water parameters. They arent super calm fish, but they arent too boisterous that they would disturb the glass catfish.
I hope this helps some.