The best rule is to ignore the language that the newly borrowed word came from and to use the same basic English rules for all inflexions, as though it were fully naturalized.
The truth is that it depends in part on how well assimilated the word is into English, and what the tradition is in the particular domain where that word is being used. Sometimes more than one version of a plural inflexion exists, such as both the English indexes and the Latin indices for the noun index, where mathematics tends to use the more conservative form.
Greek and Latin loanwords often conserve their classical plurals, which is why we have species and series as both a singular and a plural, not to mention words like genus/genera, antenna/antennae, vertebra/vertebrae, mare/maria, thesis/theses, crisis/crises, emphasis/emphases, taxon/taxa, mythos/mythoi, dictum/dicta, paramecium/paramecia, phenomenon/phenomena, penis/penes, clitoris/clitorides, crysalis/crysalides, octopus/octopodes, fungus/fungi, nucleus/nuclei, radius/radii, criterion/criteria, matrix/matrices, cicatrix/cicatrices, larynx/larynges, stigma/stigmata — and many, many more. You also sometimes see special plurals from French like plateaux, châteaux, and tableaux, or from Italian like mafiosi, tempi, or paparazzi. Those don’t tend to last as long as the truly classical ones sometimes do.
It just depends on the field. For example, in computational linguistics the word corpus comes up all the time, meaning a body or collection of texts. In that domain, the expected form of the plural is the Latin one, so you have several corpora. However, we do not (to my knowledge) use the Latin genitives unless we are forming the species name of a particular taxon. So the possessive of the singular corpus is simply corpus’s in English instead of the Latin genitive singular corporis, and the plural possessive is corpora’s in English instead of the Latin genitive plural corporum. (Again, taxa are excepted, but those aren’t really English away; they’re “modern” Latin.)
The OED attests hundreds of different singular-to-plural irregulars for imports, but most of these don’t last long in English. It might have been bruschetta/bruschette and pannino/pannini right at the start, but those didn’t lost long, such now people look at you funny if you speak of “a graffito” in the singular, and it is not uncommon to get “double” plurals, like “two panninis”.
Just use English rules on loanwords’ plurals — unless you’re working with one that is expected to retain its irregular form. Those you should be able to find in dictionary.
And the plural of virus is viruses. Always. :)