I've done that before with LFS bought fancy guppies and had no problems. Guppies bred like crazy and provided live food for the discus. The only thing I found was that because of the discus temperatures, I keep mine at 84 degrees, the life span of the guppies, especially the males were short, like 18 months
I agree with the above posts.
Yes, the guppies should be quite compatible with the discus - i've done it myself - no problems.
However, yes, the temp is generally too high for them & it will shorten their life span.
It's that's a combination of fish that suits you, go for it !