In general, mixed shoals don't work.
In some cases, where you really can't source the same species (rescues, rare species), it's better to have mixed shoal than have the fish in too small a group, but it's always, always better to have same species shoals, if at possible.
There's also the fact that, in the safety of our tanks, most fish don't school, and with a mixed shoal you end up with a rather 'messy' looking tank. Most people who've tried it regret it later.
The one exception is where fish, of the same species, come in different colour morphs (like normal/albino/green tiger barbs). Then you can mix the colours and still have a good sized group.