Opening or crushing a capsule or a softgel is generally not recommended.
The form of the supplement is designed to allow for the release of active ingredients at the right time and in the right place in the body. Modifying it could therefore reduce its bioavailability.
However, for individuals who have difficulty swallowing capsules, this remains possible. There is, however, an exception for softgels (where the contents are liquid) and Probiotics, where the capsule is crucial to protect the bacteria until they reach the intestines.
As for tablets (such as Vitamin C or B Complex), modifying them will not affect their quality and/or effectiveness. You can do so without any issues.