When a foreign term becomes incorporated into English, the English rules for plurals normally pertain. Thus, 'nostrums' is correct.
There are a few oddball words that do not take the regular English pluralisation, although they are considered (more or less) common English usage.
Sequela/sequelae is one that comes to mind.
Bill (who once won $5 betting an English major that the word 'seriatim' was considered proper English rather than a foreignism. Love that OED)