First, I'd just like to state that this topic is in reference to potted plants, normally I'd never consider fertilising an Acacia planted in the earth unless it was showing obvious chronic deficiency symptoms. I live in Australia and the native rhizobia & mycorrhizae that colonise the root system of Acacias planted in soil can provide all the nourishment the plant needs.

I've got some simplicifolia & confusa (not exclusively tropical plants) seedlings and I'm wondering if phosphorus sensitivity is an issue with these species.

Many Australian Acacias and natives are sensitive to phosphorus and it has been shown to kill them.

I think for many Acacias phosphorus sensitivity isn't as problematic as Proteas, some Banksias and some others, but I have heard anecdotes of people killing their prized seedlings with too much phosphorus. I've used miracle grow on maideniis before with no problems, but I'm not willing to test it on any of my top shelf plants.

Maybe some of the northern hemisphere members have more experience in fertilising Acacias with regular balanced NPK ferts ?

Here in Aus. it's almost common knowledge about phosphorus sensitivity in native plants (although this sensitivity is often overstated) and we have many low phosphorus fertilisers to suit.

I found this site Phosphorus Needs of Some Australian Plants which lists sensitivity levels on a score of 1 to 7 with 7 representing species that are highly intolerant to phosphorus. The list is not exhaustive by any means but it does give a good oversight of sensitive species.

I noticed in the listings for level 6 tolerance/intolerance that neurophylla an victoriea get a mention. These are two of the stars from nens thread The TOP 8 ACACIAS to Grow WORLDWIDE so I'm starting to wonder if phosphate intolerance has any correlation with alkaoid production - this is just baseless speculation though.