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You should't use a hard finish over a soft finish or is it a soft finish over a hard finish?
I think it's you should't use a hard finish over a soft finish, but just wanted some confirmation.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Don't use hard over soft.

Colin, it is fat over lean. The softer, or more oil/less resin varnish will give some over a harder, denser finish. The other way around you can get more cracking. Oil, poly; over shellac.

Some people, probably more in the violin world, go for the crackle when going for an antique look. I would think that you wouldn't want it crackle all over though. The ones who do that know what they are doing.

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Why be normal?

Some violin repair people will apply a thin coat of French polish (hard) over an oil varnish (soft) to improve the look of an old finish. They can get away with this because the oil varnish has fully cured and is "moving" less.
It is also possible to apply a - thin - coat of a drying oil finish (soft) on bare wood to intensify the color, allow it to cure for a few weeks and apply a hard finish (e.g. shellac) over top.
"Softer" finishes tend to expand and contract more than "hard" finishes, and as Barry and Ken mentioned it is generally not a good idea to put a hard finish over a soft finish.