I believe you could try adding some guppies, and see the results. Remember there is always a chance for things to go wrong, so do it at your own risk.
You could consider adding a more secure species in there: maybe a pais of otocinclus, of some corydora catfish. They will not harm your small neons, and will compliment your tank set up very nicely.
I'm not a guppy fan myself, so I wouldn't risk a good neons set up, but those are just my two cents.
If you decide to add more fish, remember to quaretine them for some weeks. Neons can be fragile to many diseases, including ick, which is very common.