Choosing Variant Rules
Source GM Core pg. 82
If you're not sure about a variant rule, take a chance! Make sure everyone in your group understands that this is a trial run and that you might need to adjust or remove the variant rules later on if they're causing unexpected side effects or not working as you intended. When you're playing with variant rules, be sure to let any new players who join the group know about the variant rules your group has chosen. This helps them set their expectations, which is important for making sure there's a feeling of fairness among your players.
If your group is playing a game with themes that call for it, you might wind up combining multiple variant rules together, possibly applying several options at the same time. For instance, in a gritty, hard science fiction or survival-horror game, you might start the PCs as 0-level characters and alter the proficiency bonus progression to remove level at the same time. In general, the variant options in this chapter are sufficiently self-contained, with explanations of how they change the game, that you should be able to combine them without trouble. When you design your own variant rules, be on the lookout for places where new rules might have unexpected overlapping effects on each other and the game.
If your group is playing a game with themes that call for it, you might wind up combining multiple variant rules together, possibly applying several options at the same time. For instance, in a gritty, hard science fiction or survival-horror game, you might start the PCs as 0-level characters and alter the proficiency bonus progression to remove level at the same time. In general, the variant options in this chapter are sufficiently self-contained, with explanations of how they change the game, that you should be able to combine them without trouble. When you design your own variant rules, be on the lookout for places where new rules might have unexpected overlapping effects on each other and the game.