I done patterning tests with my Rem 870 and 11-87 and here is what I came up with for your applications.
For your slugs - you want an open cylinder (improved cylinder the tightest, but cylinder bore better). A rifled cylinder bore choke would be better if you can get one.
For waterfowl - I purchased full chokes designed for steel and patterned these along with improved cylinder and modified using 3 1/2" loads of BBB, BB, 1, 2, 4shot steel, Rem Heavy shot in BB and 3" 4, 2, 1 and BB steel.
Out of the 3 chokes patterned, the Remchoke modified choke gave the tightest pattern in all loads, followed by the Remington Steel full choke, and the improved cylinder Remchoke.
For pass shootingor decoying geese I'd recommend 3" BB or 3 1/2" BBB or BB steel (1 shot will be OK if not too high). Pass shooting ducks 1, 2 steel in 3". And for ducks over decoys - 3" steel in 2, 3 or 4 shot.
For big Canadas I like to keep my shooting to 50 yds max. with 3 1/2" BBB.On a clear day I won't shoot unless I can clearly see the white cheek patches. That's my indication that they are in my range.