kenshin620 wrote:Terrain is terrain, use the BRB to determine its effectiveness
This.
I have the Aegis defense line, but I haven't painted it yet. For normal games, I'd use the shield walls as impassible terrain for vehicles, difficult terrain for infantry. I'd say the infantry has a 4+ cover save while behind one or more of the walls. However, I would make sure that the table is balanced by either splitting the pieces into a wall on opposite sides of the table, or put a similar terrain piece on the opposite side.
It's up to you and your opponent to decide these things, like you should with every terrain piece before the game begins.
As for the flak guns, I don't have Planet Strike, but they might be covered in that book. Addition counts as:
Imperial Guard Heavy Quad-Launcher:
IA-A2
Hydra Flak-Tank:
Apoc (Not sure if it will fit on a tank chasis).
Imperial Sentry Guns
IA:A2
Imperial Support Weapons Platform
IA:A2
I don't have these books in front of me right now, but the gun could be a reasonable stand in for one or two of these, I believe.