filter would be for mechanical filtration, not biological. However, yes I think it's a good idea, so you can run media too.
As for flow, you are aiming for maybe 30x the turnover rate in a FOWLR
30 x 40= 1200 Gph
, so you are basically on point.
As for your rock, it doesn't seems to be too pores, which is fine, but a more pores rock would have been better. This will leave plenty of swimming room for fish.
The powerhead placement is fine, however I like to point them at each other to create a random flow pattern.
I also noticed you are using Dr. Tim's to cycle, what day are you on?