Learn English – Individual + plural or individual + singular

Individual is indeed a noun and an adjective. When used as a noun, it refers to a person, or someone. For example:

That individual over there is a nasty piece of work.

To give a couple of examples of the use of the adjective, using heartbeat in its singular and plural:

Singular

You can feel each individual heartbeat when you take someone's pulse.

Or

When I put my hand to your chest I can feel every individual heartbeat

Note: when singular, individual would be preceded by each, every, one, an, or the.

Plural

If you add up all of the individual heartbeats, and divide them by the time you felt them in, that will give you the person's heart rate.

Summary

So, in short, use individual heartbeat if you are talking about one particular heartbeat, or individual heartbeats when referring to the separate heartbeats as an ensemble.

In the example that you provide,

Morphological and statistical features of individual heartbeats are used to...

you are referring to the features of a collection (plural) of heartbeats (even though, you are considering each and every individual heartbeat), and so you are correct in using individual heartbeats