What beliefs would we consider reasonable for a self proclaimed Christian to hold?

[drfuzzy]

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At 500 individuals
Humanity would need to insure genetic diversity.  Monogomy would be counter-productive.

At 50 individuals
Each individual would have to have as many babies with different partners as possible over their lifetime, and the survival of humanity is still at risk.  Lose 12 to injury, illness, or childbirth, and the human race goes extinct.

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The Biblical account is the correct order for life existence:

(Old Testament | Genesis 1:20 - 22)

20 And God said, Let the waters bring forth abundantly the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven.
21 And God created great whales, and every living creature that moveth, which the waters brought forth abundantly, after their kind, and every winged fowl after his kind: and God saw that it was good.
22 And God blessed them, saying, Be fruitful, and multiply, and fill the waters in the seas, and let fowl multiply in the earth.

This was done over a very long period of time. Adam and Eve had a lot of children over their long lifetime on the earth. After the flood the lifespan of man was still much greater than today and families were large. The Bible does not have a timetable. Suffice it to say that it took the necessary time needed for the creation to take place.

There was no "Adam and Eve".  There were no variants of humans, ardipithecus to homo sapien - that lived extraordinarily long lives.  There was no flood, it is scientifically impossible.  You really need to study something with real data instead of a silly book of bronze-age fairy tales.