The "regular Gammarus" (scud - aka shrimp) and one that is most often referred to here is Gammarus lacustris, a larger individual that grows primarily in cold or cool water habitats. It does not like water that gets into the 80's. So stocking this one with the warm water grass shrimp will not likely be productive. It would be like stocking trout(cool-cold) with tilapia (warm).
Quite a bit of past discussions about shrimp and scuds:
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=140120http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=132273http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=18046http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=18024However there numerous species of Gammarus scuds some that will live better in warmer water weedy habitats. There are about 80 species in 8 genera. Many of the species live in caves or dark places. Identification to species is often difficult and requires careful very close examination of specific body parts. Some of the Gammarus type scuds are small about 1/4" (Hyalella azteca) and some are larger 1/2"-3/4" G. lacustris. Some of the larger species are Gammarus fasciatus, G. lacustris, and maybe some types for sale on the internet where they are cultured in warm water (see below).
This link lists all the known species - there are a lot of them; so do your homework before wasting your money stocking quick snacks for your fish.
http://en.wikipedia.org/wiki/Gammarus G. fasciatus & G lacustris
Many scientists categorize them as cold water stenotherms, meaning that they require a narrow range of cold temperatures in order to survive (10-15�C, with temperatures of 20-24�C being tolerable).
The length of time G. fasciatus can tolerate a specific water temperature above 20�C(68F) decreases with increasing temperature. Temperatures of around 34�35�C and more cause relatively rapid mortality. G. fasciatus is frequently associated with thick macrophyte beds.
http://animaldiversity.org/accounts/Gammarus_fasciatus/Warmwater scud for sale:
http://www.ebay.com/itm/Gammarus-Shrimp-...e-/121295137554 Notice how fast the sunfish eat the scuds when dense weed cover is not present. Scuds are very vulnerable without dense weed cover; weed cover that is too dense for anglers to fish in is where scuds thrive .
https://www.youtube.com/watch?v=pLbiKQGoRX8 
