"Ten minutes is all it takes." — "Ten minutes" is plural, but "is" singular. What if we use "a few"?: "A few minutes is all it takes."

Question

I know the rule that if "number + measure" is perceived as one whole, we use a singular verb.

Here are some examples [(1a-5a) are from the internet; (6a) is mine]:
(1a) Ten minutes is all it takes.
(2a) Twenty miles is a long way to walk.
(3a) A hundred dollars is enough to buy the drugs.
(4a) Fifteen grams is enough to kill you.
(5a) There is five kilograms of eggs.
(6a) Three kilograms is what we need.

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Will "is" remain correct with "a few", or must we use "are" instead?:
(1b) A few minutes is all it takes.
(2b) A few miles is a long way to walk.
(3b) A few hundred dollars is enough to buy the drugs.
(4b) A few grams is enough to kill you.
(5b) There is a few kilograms of eggs.
(6b) A few kilograms is what we need.

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Will "is" remain correct with "many", or must we use "are" instead? (more precisely, "so many" because "many" without "so" doesn't seem appropriate to me here):
(1c) So many minutes is all it takes. — This sentence probably doesn't make sense.
(2c) So many miles is a long way to walk.
(3c) So many hundred dollars is enough to buy the drugs.
(4c) So many grams is enough to kill you.
(5c) There is so many kilograms of eggs.
(6c) So many kilograms is what we need.

Answer 1

All your a and b examples are correct except 5a and 5b. When you're saying how much of something you have, we don't treat it as a whole, and we use normal singular/plural verb agreement. So they should be:

(5a) There are 5 kilograms of eggs.
(5b) There are a few kilograms of eggs.

"So many" is not idiomatic in any of the c examples. They're wrong whether you use "is" or "are". "so many" is used in the following ways:

1. To emphasize a large quantity of something:

There were so many people in the elevator I could hardly move.

2. As an idiom to indicate an unknown, but limited number of things:

The economy can only support so many farmers.