The first thing to say is this,, "never" lay a floor screed onto "any" dry surface,( uni-bonded or not),, always screed onto a wet base. When i lay a floor screed, i wet the the surface of the concrete sub base with clean water,, just poured on and brushed evenly (using a sweeping brush), and then mix up a runny cement grout, (stirring it regularly cos it settles) and pour this over an area of the wet floor to be screeded, again, brushing it evenly across the floor. Just do a few sq yds at a time, because you dont want it to dry in, before you screed onto it.
If you go down the uni-bond route,, the same applies, wet/brush the floor evenly, (don't overwet), and then brush the pva mix evenly over a small area as you go,, then screed straight onto it while it's wet, (again,, not overwet), or it will make the screed mix too wet, by soaking up into it.
Make it a 3 to 1 "semi dry", sharp sand and cement mix,, and yes, fibres in a floor screed nowadays is common,, but you don't really need any other additives.
Roughcaster.