Do you know if their is any adverse affects to introducing different types of cory cats? Lets say 3 peppered and 3 emerald.
The quick answer is no, there are no adverse effects. I am assuming you are only referring to the true
Corydoras species; there are some related species that were classified taxonomically in
Corydoras but are now in
Schleromystax, and these are a different story.
As others have correctly mentioned, it is best to have five minimum of a species. However, this is not mandatory (always provided there are other corys present), and there are times when it may not be possible. I have over 40 corys in my 115g, and I have 10 of one species, 7 of another, 6 of another, 5 of another, and then fewer (1, 2 or 3) of a few other species due to only finding a couple in the store or whatever. While none of us can say how the fish "feels" about this, I am not at all certain that it is particularly detrimental. However, there is a distinction between having no option and deliberately selecting fewer than five of the species.
As you have a 36g tank, I would aim for five of the present species, and then five of the second (you have space). Wait until the ammonia/nitrite/nitrate issue is resolved obviously. And when acquiring shoaling fish (= fish requiring a group of their own species) it is always preferable to acquire them together if you can. There are various reasons, and with corys we do not need to worry about hierarchies and territories which some species do involve, but the larger the number the quicker and easier the corys will settle in to their new home. Safety in numbers to the fish.
One other point, on the "emerald"...is this
Corydoras aeneus or
Corydoras splendens? Common names are very confusing at times (almost always actually), and here it makes a difference.
C. aeneus is a normal-sized cory, but
C. splendens gets considerably larger. This latter species was previously classified in
Brochis as
Brochis splendens, but recent phylogenetic work has moved it into
Corydoras for the present. It is peaceful and gets along with other corys, but it does get larger. And given its uniqueness, I would definitely get five of these. It is actually not that closely related to
C. aeneus, or
C. paleatus (the "peppered") so it would appreciate a group of five minimum. For that matter,
C. aeneus and
C.paleatus are also not closely related, and once the phylogenetic and taxonomic work is finished both these species will almost certainly not be in
Corydoras, for reasons I won't go into unless asked.
Byron.