[Peter Finke]

Jacob, generally speaking you are right that chocolate gouramis and licorice gouramis have the same needs for their environment. I have seen tanks successfully keeping both gouramis together (but not their offspring, and that’s a pity). Your idea with the peat filter is a good idea, but take care to select the right peat; there are quite different qualities. Some are rather useless since they are pH-active to a too-low extent only. Some are dangerous because they are mixed with nutrition materials for gardening. Ask the dealer; that must not be the case! And have a test before: there must be a clear reduction of pH if you test with very soft water. (Hard water will be reduced in hardness a bit, but the important effect on pH is only a low one.So,only very soft water is reasonable). Therfore the reason for using such a peat filter system is to reduce the pH considerably in order to reduce the load with germs.
I cannot tell you in detail about Sphaerichthys; yes, they can be rather aggressive but that mus not be the case. But anyway, here is the issue Parosphromenus and they certainly will have a profit from your peat filter arrangement. Parosphromenushowever are mostly less delicate and sensitive to disease than Sphaerichthys are. Their sensitivity to diseases are well-known.
But you must obey three things:
1. generally use the softest water you can provide; nearly no hardness should be measured: “a water near to destilled water”.
2. you must change the peat regularly when the pH begins to rise considerably (so you have to control it). But beware: if you have that extremely soft water and very pH-active peat you need little amounts of peat, regularly changed however.
3. you should try to create stability. It does not suit the fish, neither Parospromenusnor Sphaerichthys, if there are rapidly changing conditions. If you have the right conditions and not many very active sumerged plants, then sometimes a permanent filtering with fresh peat is not necessary.But that you must try out.
By the way: There a measuring kits for measuring the germ content of aquarium water. These are very useful for the friends of blackwater fish, indeed.