Learn Finnish: Minun kummisetäni on lyhyehkö ja äreähkö mies. - My godfather is a rather short and somewhat grumpy man.

To add the -hko/-hkö suffix, you must attach it to the word's vowel stem. For words ending in -ut/-yt like lyhyt, the stem changes to lyhye-. Then you just add -hkö to get lyhyehkö (rather short).

Why do both adjectives take the -hkö ending instead of -hko?

Vowel harmony! The front vowels ä, y, and ö dictate that the suffix must also use front vowels. Both lyhyt (stem lyhye-) and äreä contain front vowels, so they take -hkö. If the words contained back vowels (like a, o, u), they would take -hko instead, such as in vanhahko (somewhat old).

The English translation says "rather short" but "somewhat grumpy". Do lyhyehkö and äreähkö express different degrees of moderation?

Not in Finnish. The suffix -hko / -hkö consistently just softens the adjective, meaning "somewhat," "rather," or "fairly." The different English words used here just make the sentence flow more naturally, but the Finnish suffix holds the exact same weight for both words.

Why is it Minun kummisetäni with both the word Minun and the suffix ?