That’s cool, then maybe mention what applies in the case of niche applications and what your idea behind it is. And be upfront about the costs?
Right now it sounds more like a threat: -> If you develop a niche application, you cannot publish it and will have to get in contact with us so we can tell you that you need to pay 50% of your profit to us!
How about you just state some figures for niche applications and then mention that if the developer contacts you, he may get special rates?
Do it like this:
Indie License
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Any application that sells for less than x $$ or x $$ a year to end-users falls under the indie category. In this case you do not have to pay anything.
Commercial License
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Any application that sells for more than x $$ or x $$ a year to end-users is considered to be a commercial application and requires commercial licensing. Any released app falling under the commercial license costs either xx $$ one time fee or xx % of the profit. Special license rates and help on marketing can be negotiated on a one-by-one basis, please contact our support department for help.
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And then just delete the whole “niche application” nonsense.
You have two licenses, everyone can see them and knows what to expect, no shady “we wont tell you before you contact us personally”, everything easy, painless and transparent.
How does that sound?