At campaign startup DCG allocates all squads to airfields closest to the frontline. You can suppress this by checking the "No squadron transfer" option AND reallocating them one by one to other airfields via the Timetable.dcg file. The syntax is
19420505 MoveSquad USA_BG_043z0 12_Miles_Airfield
As to the other issue, it may have something to do with pilot skill and engine management. You say it was always #5 who rammed #4. In DCG, #1 and #5 are the most experienced pilots, while #4 and #8 the least experienced ones. Both flights have to fly along the same flight path (same waypoints and schedule times), and it's up to the AI to keep a safe distance between flight A and flight B, that is, as it seems, between #1 and #5.
Now if the planes are overloaded, and/or are forced to climb too steeply, and/or you specify a cruise speed too high for them, #4 will not be able to keep up with #1, while #5 will be. Just a blind guess.
Anyway, I have had several similar problems in my campaigns, and it was my experience that most of them could be resolved by
1) reducing the cruise speed to 280-300 kph for fighters and to 240-260 kph for bombers (BoB and early Med-era);
2) reducing bomb load (particularly when you see AI bombers crash on takeoff);
3) reducing fuel quantity (in RL there was a trade-off between bomb load and fuel allowance; btw, on most Il2 maps, you don't need 100% fuel, not even for Bfs).