Hi,
if you want to breed amphipods as food, I'd recommend to breed Hyalella azteca (Mexican scuds).
They are much more hardy than European amphipods, they don't need as much oxygen as Gammarus pulex and similar species and they reproduce much faster, too.
They are smaller than Gammarus pulex, around 8 mm (0.3 in) maximum and they need higher temperature to reproduce. But if you want to breed them indoors that could be an advantage, too.
As they really eat everything, they can be dangerous for eggs and very small larvae of newts. So I would not recommend to introduce them into a tank with eggs or recently hatched larvae.
One other disadvatage is: If they are introduced into a tank with lots of hiding places, they will reproduce and it will be very difficult or even impossible to get them out there ever again.
And if it is a planted tank they might eat the plants then.
There is also the danger to introduce them unintentionally to other tanks by moving plants or other things from one tank to another. So plants from a tank with Hyalella azteca should not be used in other tanks unless you want to have these critters there, too.
It should be rather easy to get a colony of Hyalella azteca. At least here in Germany these "Mexikanische Kampfkrebse"

are quite common as food for bigger fish and newts.
Here you find some information about breeding this species:
• View topic - Pardon my Gammarus!
And some additional information:
Hyalella azteca - Wikipedia, the free encyclopedia
Peter
P.S.: If you breed them, then please never let them get into natural waters like ponds and streams. They can be very dangerous for native critters like Gammarus pulex and others.