well you can make a mixed reef as hard or as easy as you want to... but it's a great starting point to learn the basics...
generally, the ones that need a species tank are either very time demanding and finnicky/delicate, like seahorses, or dangerous, ie. lionfish. we all get bit or stung the first year or so we do a mixed reef... better it's just a clownfish nip or a frogspawn sting than a poisonous lionfish spine, until you become more advanced.
i mean, if you've DONE a mixed reef, and are looking for a new challenge, go to it... if not, man, do a reef, they are SO diverse in life, and interesting to watch... and like i said, you can make it as hard or easy as you want, depending on the creatures you pick.
lsctorc makes a good point about the size of the tank, to start, bigger is better.