Freshwater / Tropical fish dilemma!

Right, after massacring 2 goldfish, 3 fantails and a black moor during my
abortive first attempts at amateur fishkeeping, I've done all my homework
and am now pretty much ready to start over again.

I'm using a 21 litre tank, and have been trying to cycle it for 2 weeks now
by adding cycling bacteria, a plant, and putting food in there to rot and
produce ammonia. Thus far there are no nitrites at all, some nitrate (which
I assume is a good thing, as it shows the bacteria are working), although I
don't have an ammonia test kit.

Anyhoo, I'm going to be patient and take it easy, and am probably going to
buy a fish tomorrow, most likely something small that won't grow huge and
will be happy in a relatively small tank. So the questions I have a

a) Any suggestions for a good starter freshwater fish that is pretty durable
and won't grow to a huge size? I want something quite pretty so am prepared
to pay a bit over the usual bog-standard prices....

b) I'm considering buying a heater and getting some tetras or tropical fish:
is this advisable for my first proper tank? I was trying to think of the
extra hassles that heated tanks would bring, but can't really think of any.
What are the pros and cons? I assume you can add cold water during 10% water
changes....

Any ideas would be much appreciated - determined to get it right this time!

R