I disagree.
many small catfish will do fine.
Neons can be kept at room temperature; this means you can safely add small Glyptothorax, Gagata spp, Hara jerdoni (Deltawing catfish) or other critters. Most are hard to get and some are not that easy to keep. H jerdoni however are fun, as long as you don't boost the heater.
Other Asians that need usual temperatures (22-25'C) that are also harmless with small fish include Pseudetropius brachypopterus (wich often pops up as "silver Pangasius" in the LFS), Neotropius spp, Hyalobagrus flavus (usually called Pelteobagrus in the trade, a very nice harmless dwarf catfish), Orange Banded Akysis spp and of course the small, pretty True Glasscatfish, Kryptopterus minor, a harmless shoaler.
if those Asians are not your cup of tea, you might think about small Africans.
Any Microsynodontis can be kept in that tank, the same goes for Synodontis contractus, S nigriventris, Mochokiella paynee, Chiloglanis spp, Amphilius spp, the popular shoaling African Striped Glass catfish: Pareutropius buffei (often sold as "eutropiella debauwi") and the more fragile Parailia species wich are best left for experienced keepers.
The list of South Americans goes on and on....
ANY Aspidoras, Corydoras or Brochis will do; however: best stick to a single species, keep a group, and avoid the bigger species. Just check the catelog; these genera alone supply you with literally hundreds of choices, Cory-cats are popular, and available everywhere.
Harmless too are both Dianema species. Despite their slightly bigger size, they're docile, harmless groupers. The same goes for the small (8 cm or so) Lephoplosternum spp (usually imported as "Hoplosternum pectorale"). None of these will harm Neons, and most can be kept with little difficulties if any.
Small Pims might snack on your Neons, really small Auchenipterids won't: the beautiful and highly appreciated Centromochlus perugiae is one of THE prettiest small Amazonian non-cory-cats. It'll setlle for small tanks, just add some dense bushes and some bogwood. Rewardingfish, but shy.Entomocorus benjamini is less striking but as docile. Other Centromochlus or Tatia are often by-catch and similar sized. Trachelyichthys exilis is another pretty that should not eat fish, except for fish the size of baby guppies.
Small Banjo cats can be a LOT of fun! Bunocephalus verrucosus (often still called "Agmus") is completely harmless and well suitable for your tank. Most other available Banjo's are too.
Small Doradids should be fun too (Opsodoras, small hassar etc). Unfortunately, they're very, very rare.....
Loricariidae: This list goes on and on and on.
To stick with kicking in the open doors:
Hemiloricaria spp (these are usually sold as "rhineloricaria" or plain "Loricaria") with H lanceolata as best example; Farlowella spp, Otocinclus spp (with the easily available O vittatus and O macrospilus running up front), Parotocinclus spp, Chaetostoma spp, Ancistrus spp, Peckoltia spp, Sturisomatichthys leightoni.......
and I haven't even started on the smaller L-numbers!
All the Twiglike suckers do fine single or as a pair or small group, the tiny Oto's are groupers/shoalers and the bigger suckers like Ancistrus etc can be a bit territorial but do fine as pair or single.
To top it off, if you ever see Trichomycterus spp anywhere, go and get them. They're small, they're fun, they're harmless, very rare and easy to keep as well! I've kept T alternatus in the past, and they're in my top 10 of "most fun to keep catfishes if you have only 1 small tank".
sid