Translation:I eat English breakfast off the table.
"micul dejun" already has a definite article, so you can't prefix it with the indefinite "un". You can, however, say "un mic dejun".
Yes, "a lua micul dejun" is a valid expression. "dejun" is a neutral noun (masculine in the singular, feminine in the plural: un dejun - două dejunuri). The adjective "mic" has to agree with it; the plural is, theoretically, "micile dejunuri" but I've never heard anyone say that.
You could probably treat "mic-dejun" as a single word and say "mic-dejunurile" (although the dex doesn't list it).
This generally happens when the noun-adjective order is switched (the reasons for doing so are similar to those of English, mainly emphasis):
Fata frumoasă. - Frumoasa fată.
Un prieten bun. - Un bun prieten.
Speranțele mari. - Marile speranțe.
Castelul vechi. - Vechiul castel.
O șansă nouă. - O nouă șansă.
However, not all adjectives can be placed before the noun and I couldn't find a reliable source that offers a satisfactory rule. The link below claims these are those adjectives related to color, shape, social/religious/technical status, or a state...
Aaaaand, there are a few adjectives who can only go before the noun, such as "biet" ("poor" as in "deserving pity"), "sărac" (when used with the same meaning as "biet") and some pronominal adjectives: fiecare (each), orice (any) etc.
This article in Romanian seems pretty decent and touches the topic of noun-adjective order in section 4.