Research suggests that individual differences in additional language learning may play a more important role in taxing situations when learners are confronted with unfamiliar or difficult tasks. However, studies to date have mostly focused on second language (L2) learners/bilinguals, while individual differences within multilingual populations remain under-researched. Working with university-level multilingual adults, we compared the effectiveness of traditional instruction (familiar) and concept-based instruction (unfamiliar) to teach the past tense/aspect distinction in Spanish. Learners were pre- and post-tested on their knowledge of the target structure and assessed on language learning aptitude, working memory capacity, verbal-imagery cognitive style and attitudes. While both treatment groups demonstrated significantly improved metalinguistic knowledge, we found no statistical differences between the two groups in terms of knowledge gained or attitudes, and individual differences in cognitive ability were not associated with observed gains. This set of results indicates that the cognitive individual differences measured seemingly no longer played a significant role in these multilinguals' performance in the instructional conditions examined. In addition, neither language learning experience nor typological closeness between known languages had any significant impact. We propose that extensive experience with explicit language instruction may have led to a levelling effect, as previously observed in L2 learners.