I have a ton of experience mixing species and I would highly recommend against this combination. Grandis are some of the most curious and aggressive geckos in the genus and they do prey on small lizards/frogs in the wild. Even if the Terribilis are too big to eat they can still get injured/harrased by the Grandis.LOL ^
I know alot of ppl say don't do it and I can see the problem if you have similar species. ie; if you have Azureus and cobalt tincs in the same tank, theres a chance they may breed creating hybrids.
I don't see an issue with different species, ie. thumbnails and leucs, etc. I mean, an azureus is in no way going to breed with an imitator or a vent.
That being said I have seen multiple species being kept in tanks like NAIB and I dont see the problem. (I know the "yeah but they have the experience to do that" comments coming).
BS i say! They have the same issues that we all have, they just have a larger staff and a bigger backroom.
I currently have a trio of Azureus and a group of vents in one of my 40b verts and all are doing well. I never see the vents on the bottom of the tank, they absolutely love being up in the broms. On the flip side, the Azureus have never really climbed up the tank. They always stay on the bottom searching for springtails. Once I saw one up on a branch but it fell as they aren't the best climbers.
Bottom line, IMO, is trying to stay away from the interbreeding but if you have a large enough tank I say go for it... mix it up. My other 40b vert currently has a pair of Giant day geckos in it but soon with have a group of 6 terribilis and I can just hear the flames coming on that one but I don't see the issue. Ther terribis are big, the day geckos wont eat them and everyone will be happy.