Can I use a MacBook Pro 13-inch charger in a MacBook Pro 14-inch ?

Your computer performs best when connected to AC power. It can use the full output of the Power Adapter AND when doing especially challenging work will also freely "borrow" power from the battery. In some cases, even with the power adapter connected, the charged state may decline during stressful work.

if you are using a power adapter smaller than the one that shipped with it, 'charged state declining during use' may be more pronounced, and re-charging may take longer than expected.

As long as the plug to your MacBook is the same, it should work. I've often swapped power bricks between machines. I keep mine at my office and use my wife's MacBook Air while at home. Same USB-C connector so they both charge fine.