@Nat093,
You can use the single quotations marks or not use them. Consider the needs of the reader.
For example, in the sentence you just wrote:
But I guess 'linking element', 'interfix', and 'intermorph' are also used as words in my sentence, and you suggest using them without quotes.
It's wonderful for me as the reader that you immediately put the words in single quotations, because without them I may have read 'linking element' as a linking element and not as a word and then I continue reading the sentence with a wrong impression. When I get along further in the sentence, I see 'used as words' and so now I have to go back and reinterpret what I read and start over.
But consider the original sentence,
This affix is often called a linking element, an interfix, or an allomorph.
You clearly and early indicate that what follows the word 'called' will be words or phrases that can clarify 'this affix'.
I don't have to go back and later figure out that you were using these words as words, because you already indicated that before you used the words.
Yes, I know that in the more complicated sentence you did the same, but those phrases are quite long and so the single quotation marks help the reader.
You are using the punctuation and quotation marks to help the reader. If your sentence clearly does that already, then they might not be necessary.
Do you need them in the sentence about 'fat' and 'grease'? Yes, you do. Otherwise the reader may misread the sentence.
Is there anyway the reader can misread your first sentence without the single quotation marks? I don't think so. It's a very short, simple sentence.
Is there anyway the reader can misread your more complicated sentence without the single quotation marks?
In literature the term phrasal verbs is alternatively used with a variety of other terms, including compound verbs, particle verbs, phrasal and prepositional verbs, two-word verbal idioms, and constructional idioms (compare Quirk and Greenbaum, 1973; Fraser, 1976; Benson et al., 1986; Johnson, 1991; Jackendoff, 1997).
To me, the possible misreading occurs here: 'phrasal and prepositional verbs'.
Is the correct term 'phrasal and prepositional verbs' or did you combine two different terms--'phrasal verbs' and 'prepositional verbs' to invent a new single term (do not invent a term) encompassing two different terms? Without the quotation marks, it would be difficult for your reader to know which is the correct phrase.
With the single quotation mark, you are clearly indicating that the accepted term as listed in the references you are citing is exactly 'phrasal and prepositional verbs'.
If that's not the term, then you need to change it to read 'phrasal verbs', 'prepositional verbs', and so forth.
If you do have to change the sentence and list both terms separately, then I can't see that there would be a chance for your reader to misread the terms without quotation marks.
Conversely, if the correct term is 'phrasal and prepositional verbs', then you should keep the single quotation marks for clarity for the reader.