Yes, after I'd found the second type, I skimmed around in the ship for a short while in case I came across any more types but without success, and as I had no idea if any more types were there I soon gave up and moved on!
FWIW, I've found up to 3 different types of brain trees in the same location.
Yeah, i've spent quite a while in Guardian space and there they're often growing on planets without atmosphere. The most i saw was also 3, seems rather rare, while 2 is quite common. It's fairly easy to scan them, as, firstly, if another variety is present, you spot it really fast - e. g. if you see a dozen of trees, they're going to be different. So not like with Ody bio - you hardly search for anything, you can see it from your ship even - if they're all red, or some green or violet grow in between them. And secondly, the distance required for the scans is very modest - it's somewhere around 200m, so you're good running on foot between them and scan. Nowadays they're paid 1,5m for a known variant, cheap, but they're very easy to spot. That's also true that heatmap only lists them as single signal, while some variants may exist. But again - if you see 10-20 trees and they're all same color, then that's the planet with only one color. They're indeed a nice source of raw mats, G2 always, G1-3-4 - if you lucky to find a right planet. All my raws basically come from there - it feels like gathering apples and i like it.
All that being said i don't know why Selenium grows in Polyporous Growth that is usually about G2 mats (and Selenium). Grey stone-like things on other two trees are for G3 (check planet composition - G3 stated there), yellow wingy flower on the left - G1. You may also see 55% shield left, but in most cases i saw those trees are peaceful and aren't attacking you even if you gather the fruits.
That's the thing that is for G4 (xcept Selenium). I have written down some planets as i checked bios while cheking Guardian ruins, but i later found this kind of intermittent. Like, on some planets, Phloem Excretion (that's the thing on that pic) can easilly be found on some of the trees, while on the others - you land again and you can only see the common Polyporous Growth. I don't have an explanation for that and can't be bothered to read guides, guess it's fine, that i know a good source of Ruthenium while for Tellurium i bookmarked smth, but i can not find it again. As evident from the pic, shield is on 100% - again, trees peaceful. They don't attack you.