First, I do not think the "in the wild, chams are usually always found solo" comment is accurate and definitely not defensible.
Next, I have personally observed numerous Furcifer pardalis living in close proximity to adult Phelsuma madagascariensis grandis while I was in Madagascar.
Now, with that said, I think the important thing to note is that captivity is not the wild
Ultimately, I would not recommend attempting to keep F. pardalis and P. m. grandis as cohabitants in captivity for a number of reasons. First, there is the chance of the chameleon eating the gecko. Secondly, some chameleons might be stressed by the presence of a large gecko in the same enclosure, although I tend to think many wouldn't be to bothered by it based on the behavior of these geckos. Finally, I don't think a chameleon cage set up for a panther chameleon is an enclosure that would allow appropriate appreciation of the gecko.