What you cannot do is use an Instantaneous Trip (IT) only breaker , aka "MCP", in anything other than a factory assembled and listed combination starter. If you build your own starter, you MUST use either an inverse time / "Thermal-Magnetic" (T-M) breaker, or now, what's called a Motor Protector Circuit Breaker, aka "MPCB". MPCBs have thermal and magnetic trips, but the thermal trips are adjustable, like an OL relay.
T-M circuit breakers are going to be molded case for the most part, but that's not actually a requirement, it's just the way the market works. You can get MPCBs in molded case format, but smaller sizes are actually what is known as an IEC version of a "Manual Motor Starter". The UL listing is under UL489, which is the same as the T-M molded case breakers. So they are listed the same, but not built the same or used the same way. MPCBs can only be used on motors, because of the adjustable thermal trips. A few mfrs, Siemens for one, make a version that has fixed trips and is listed as a T-M circuit breaker, but they are more expensive, so there's no point in using them.
Bottom line, you have more choices for motor starters, it's in applications that are NOT motor starters that you are more restricted.
