Brian, if your reasons for raising the pond "slowly" are water quality changes, no need to be concerned. The volume may be large, but the fish will more than acclimate easily as it fills, even if you used a 2" trash pump at full flow. Your fish would like it far better at full pool sooner than later!
As for filtering the lake water, using fiberglass window screening on the intake and out flow would be plenty adequate for fish, and still allow the good stuff to enter. An intake I made was a 2" PVC 'T'. I drilled a couple hindered 1/4" holes on the 10 foot long cross of the T and covered it in several wraps of the 24" screen material. The extra wraps were fairly loose to allow the entire surface to be drawn from rather than the holes drilled only...it never clogged in 60 days of use in an FA laden pond. Black vinyl tape held it on well.
