There's no real transparency, as the opacity slider is either black (below 0.5) or white (above it). But it does let more light through if you reduce opacity to say 0.8. I sometimes use that to get more light in dark tree canopies. It also lightens shadows on ground, but does so in a gradual 'translucent' way, which is not always what you want.
Take care to either use a separate opacity mask (usually greytoned simple tif or so), or the alpha mask in an RGB file containing the leaf color, which is what I always do.
I never use any maps for translucency, though some people do, and it might be better for real close-ups. Just slide it to 0.5 or so, what Kevin said. You have to look at the nature of the leaves; some just are thinner and let more light through than say needles or succulent leaves. Same with reflection. This also gives more (reflected) light, and makes a plant more 'alive'. But also depends on type of leaf.
Then you can add tint variation to leaf textures over an entire population by either using the built-in method, or through a PF and a transform shader set to world/final position, attached to color function. Or you can mix bark types by PF+transform, all to vary trees over a population.
Then save to tgo in a library folder you don't change anymore and you'll have them at hand for eternity
I've done many hundreds of plants by now.
I wouldn't really worry about size, as TG loads huge amounts of data very easily. Though it's fair enough to use lower poly trees for big expances in the distance. Balancer can reduce polycount, other software probably too. And leaf/bark textures needn't be extremely large too, mine are usually 1-2k.
