Ex pluribus unum means (simplified) "From many, one", in the sense that many parts build one whole. Can I also use the phrase in the sense that from many possible solutions or things only one (the best) is selected? It's for a tagline, so there is no further context.
One of the senses of "ex" well cited in Lewis and Short is to "specify a multitude from which something [or someone] is taken" - for instance someone is chosen from a group to do something. So I think your suggested use would be perfectly proper. It would not of course be clear that the tag referred to one of a choice of solutions: but then in the motto of the USA, "e pluribus unum", it is not stated what is being referred to (it is the name of the country that makes that clear) and with heraldic mottos it is often not explicit what is being referred to either. If you wanted readers to realise you were referring to solutions, you would need somewhere to give them more information, but this would not have to be in the tagline.
Indeed, the PHI Concordance reveals many examples¹ where ex pluribus unum means one out of many, for example:
cum praetor unum ex pluribus iudicare uetat, ceteris id committere uidetur. (Just.Dig., 5.1.12): “When praetor prohibits one of many [judges] to give a verdict, other [judges] are [nonetheless] expected to.”
si uni ex pluribus iudicibus de liberali causa cognoscenti de re non liqueat... (Just.Dig., 42.1.36) “If one of many judges does not consider the matter proven...”
quae idem significarent solitos ediscere, quo facilius et occurreret unum ex pluribus... (Quint.Inst., 10.1.7): “those in the habit of learning [words] with exactly same meaning, so that [any] one [word] of many would come to mind easily...”
¹ With thanks to Nathaniel for the reference in their answer to another question.