Hi, I have a race with the ability to Grapple as a Bonus action when they make an Unarmed Strike. They can use this ability a number of times equal to their Constitution modifier until they recharge with a Long Rest. In the Homebrew creator, it only lets me input a fixed numerical value for the number of Uses before a Long Rest. Is there no way to tie this to a modifier instead?

Also, I am home brewing a subclass and, starting at first level, they gain proficiency in either Insight or Medicine (you choose). In the Homebrew builder, it lets me add a choice of a single proficiency, or a proficiency from all the skills tied to a particular ability ("choose one skill from wisdom") but won't let me specific specific skills to choose from. Is there a way to do this?

Hi, I have a race with the ability to Grapple as a Bonus action when they make an Unarmed Strike. They can use this ability a number of times equal to their Constitution modifier until they recharge with a Long Rest. In the Homebrew creator, it only lets me input a fixed numerical value for the number of Uses before a Long Rest. Is there no way to tie this to a modifier instead?

Nope.

Also, I am home brewing a subclass and, starting at first level, they gain proficiency in either Insight or Medicine (you choose). In the Homebrew builder, it lets me add a choice of a single proficiency, or a proficiency from all the skills tied to a particular ability ("choose one skill from wisdom") but won't let me specific specific skills to choose from. Is there a way to do this?

When creating the class feature check the box at the bottom left "has options". Let's call it "Bonus Proficiency". Don't forget to set the required level even if it's for level 1 as you say.

Click to Add An Option. Call it, say, Medicine. In description put something like "You gain proficiency in the Medicine skill." Click create option. Now add a modifier for the option: Modifier Type: Proficiency, Modifier Subtype: Medicine. Save.

Now go back and add another option and call it, say, Insight. Use a similar description. Click create. Add a modifier much like before choosing Insight instead of Medicine. Save.

Now, when your character chooses the subclass, you get the option to choose proficiency in either Medicine or Insight.

Hope this helps.

The ability to tie limited-use features and traits to an ability modifier is coming later. It was left out of the initial release of homebrew subclasses and races because it was liable to change with the character sheet revamp that's coming out this month.

Thank you very much. Off to try this now.

And I'm back and it worked. Thank you very much! 

Thank you. Good to know!