Wasnt BTC cpu mined at first? How did that change to GPU? Can the same be applied to LTC with the FPGA's ?
Yes of course, but the proof of work is different... it's been designed to need fast access to large amounts of memory, so you'd be looking at an entirely new pcb - probably with memory on it - and usb likely wouldn't push enough bandwidth to keep up with 1 (and certainly not 5) fpga ltc miners.
No, thats not correct. A single LTC hashing core needs only a little over 128
kB for working. Thats why it can run 'fast' on modern CPUs with enough on chip Cache. The fact that LTC mining with GPUs needs so much money comes from the way GPUs work. (stream processing), so there will filled a large block of memory with task to compute since GPUs are Vector engines that don't work well with branching (and loops).
I am not really sure how much memory the EP3SL150 has, i think the would be 2-4 Hashing cores per chip possible.
All in all it should be possible to construckt a bitstream but it would be not so efficient as for BTC-hashing.
BTW, bitfury did some calculation on LTC Asics, but i have no idea about the state of the project.