Advice: Using any main stat to make a roll with any skill.

Basically the title. Im not sure how to get the roll20 sheet to do this for me. I would like to give my players the option to, say, blend in with the crowd using a Stealth/Charisma roll, rather than a Persuasion or a normal Stealth/Dexterity roll. Is there a way to change the standard, compendium-compatible sheet to do this?

Not on-the-fly. You could set up a proficiency in the proficiencies tab called  Proficient and Expert, and set them to query. Then if they are proficient in stealth, ask them to make the roll you describe. They would make the "Proficient" proficiency roll, and choose Charisma from the resulting dialog box. Same procedure for Expertise with an Expertise setting. For non proficient skill checks, just ask them to make a raw ability check.

You might want to mention your game system and the sheet you are using.

I had assumed by "standard, compendium-compatible sheet" that the D&D 5e by Roll20 sheet was meant. It could be the Shaped, but it would be nice to have confirmation.

Well, there is also pathfinder.  And they are working pathfinder 2, so paizo definitely has some compendium compatible sheets. 

True. Given the latest Orr Report, I felt confident, though. :)