I'm pretty sure those dimensions you just gave are incorrect.
I'll go by the 15 gallon volume instead.
Are you aware of the inch per gallon guideline? This is useful when doing calculations for small tanks/fish like this.
Basicaly, for every inch of fish (take ADULT SIZE into acount), you need 1 gallon of water.
4 zebra danios - approx. 5" (assuming each will get to about 1.25", not including tail). Should add that zebra danios should be in groups of 5 minnimum in at least a 20 gallon tank because they are extremely active. However, they'll be ok in this tank as well provided it isn't a tall or fancy-shaped one. Also happens to preffer the upper layers which is the area preffered by dwarf gouramies.
2 black widows - about 4" total. Again, a schooling fish which should be ing roups of 6 minnimum. It also happens to be quite nippy which isn't good for gouramies' feelers.
1 lemon tetra - 2" - another schooling fish that should be in a group of 6 minnimum.
1 scissortail - 3" - again, a schooling fish and probably one a little large for this tank and more suited to a tank double this size.
In total you have around 14" of fish. If you consider the displacement of water due to ornaments, gravel etc, this pretty much means your tank is fully stocked (by the inch per gallon guideline that is). I should add that this guideline is not set in stone but it's best to keep the tank under-stocked than to risk over-stocking. If you did have room for more fish, I'd sugegst adding more lemon and widow tetras and some more of the scissortails. Right now the most I'd add is another 1-2 lemon tetras or 1-2 more danios. Keep in mind that, as your tank matures, it'll be easier to add some more fish without upsetting the balance. For now though, stick to what you have.
As for adding the dwarf gouramies in aprticular. Well dwarf gouramiES - plural - wouldn't be a good idea anyway (regardless of tankmates) as males are territorial and you can't usualy find many of the duller females available in shops anyway. Assuming you did find some, a trio would have been an option (but isn't) had you had the space. By a trio I mean 1 male with 2 females.
A single male, on the other hand, would have worked even with quite a heavily stocked tank provided the tankmates were suitable. In your case, the danios compete for space and the black widows are nippers. In a mroe spacious tank these fish could all still get along but, with such limmited space, it's risky and best avoided.
Having said that, it's never a good idea to add a dwarf gourami to a tank that's either over-stocked or very small. In both cases, the environment is likely to be unstable and, dwarf gouramies being quite fragile and susceptible to disease, don't take to this kind of thing nicely. They tend to get extremely stressed and then die suddenly.
So realy, to answer your question (and my apologies for the rant but I wanted to explain the reasoning behind my answer), no, don't get any gouramies and you'd be best off not getting any other fish at all - except maybe 1-2 lemon tetras OR 1-2 more zebra danios.
If you ever up-grade your tank, ask again. The answer will be different. Good luck.
