No no, it wouldn't be bad. There is enough room for both species in a 20 if you keep both of the schools down around the minimum(6-ish, depending on filtration and plants you could get away with an extra one or two added to one of the schools), but of course schooling species always appreciate more of their own kind, and while a 20 gallon is a wonderful size with lots of options it is still not all THAT huge.
Its completely up to you though. You can do both species without any troubles, just as a personal opinion/preference one larger school is a little better then two smaller ones. ;)
I haven't heard much about Aquaplus I'm afraid, though I know not all bottled bacteria works well/at all. Even the brands I mentioned can be a hit or miss, but I have heard some good reviews on the Stability and Safe-start, so if you wanted to give the bottled bacteria a shot I'd suggest snagging one of those to try instead. From what I understand, it won't hurt to try.