Will geese decoys attract ducks (and vice versa)?

[Macdrizzle]

I am very new to waterfowl hunting and I’m looking for some tips on decoys from you experts on here. I scouted a couple of fields/ponds Id like to try this fall season and I am looking at purchasing a dozen or so decoys. I am looking to get mainly any of the duck species that are legal to harvest plus some Canada geese and/or specklebellies. My questions are:

1. Is there a “common” decoy that will attract all the species or will a mallard decoy only attract mallards, Canada geese decoys attract only Canadas, and so on?
2. Same deal with calls, do I need a goose call specifically If I want Canadas or will a duck call work on geese? Any recommendations for a beginner's call?
3. Is a dozen decoys a good number to start?
4. If anybody is looking to sell their decoys I’d be interested in buying them off you please DM me.

Thanks in advance!