Is a SRAM necessary in a cycloneiii?

Wouldn't it be more precise to specify the SRAM needed to be effective ?

a Sram could be >100nSec slow and a sdram clock with ~125MHz faster

but an SRAm could also be very fast <12nSec .....

or do you mean the internal memory blocks used for cache, tightly couple memory and other high speed memory stuff ?

to answer his second question, yes of course sdram can be used for program memory but be aware that you need some other memory where your program is stored, nios must boot and copy your instructions into sdram and then execute from there.

for the last question, you can add as much memory devices as possible depending on available pins and others criteria. so no problem to add flash, sdram, nvram, and other external type of memories at the same time.