You need a wide range derailer. I have a 8 cog megarange cassett on my pedal recumbent and it works flawlessy. I do not have index shifting I use barend shifters. For me they work much better than index. Maybe because I have a very long chain, 2 1/2 regular chains. My bike is a toureasy clone. Here is a link to harris cyclery with info and a range of derailers from expensive to cheap. Campagnolo, Shimano and SRAM Bicycle Rear Derailers from Harris Cyclery
I woyldn't mess with the lowend ones, get the best you can afford, you will be glad you did in the long run. Hope this helps.