I have never used that mix, but the manufacturer's web site lists the ingredients this way:
Contains: Fir Bark, Lava Rock, Canadian Sphagnum Peat Moss, Sand and Aged Redwood
and assuming they are in order, there's a fair amount of lava rock in the mix already. You can probably get a better sense of this by looking at the mix first hand.
It's probably a good idea to use a final mix of half organic, half aggregate. That's what I use for my succulents. I would advise adding however much rock (perlite, pumice, lava rock, coarse sand, whatever you prefer) would be necessary to even out the amounts of organic and rock in the final mix.
I prefer pumice to perlite, but some people don't have the option, as pumice may not be available everywhere locally. Perlite is usually associated with a nasty dust which you don't want to inhale (use a mask while handling this material), and that's the main reason I don't like it. It's lighter than pumice, traps a lot of air even when wet, and has closed cells (while those of pumice are largely open to water). It might tend to blow away if you use it as top dressing. Otherwise the two should be interchangeable in soil mixes, and you should be able to get good results with either.
