Latin Dictionary and Grammar Aid: http://www.archives.nd.edu/cgi-bin/l...mmun&ending=is

immunis -e [without duty , free, exempt]; in gen. [not working or not contributing; not sharing in, devoid of; stainless].

Immunis, e, being an adjective, is therefore used to characterize something. In this case let's assume miles immunis, which means "exempt soldier". The "e" denotes use with neutral nouns, e.g. "praemium immune".

"Tirone" is the ablative (or dative) of Tiro, which has genitive tironis and plural tirones. Tribunus is a second declension noun, and therefore, Tribunus, Tribuni, with Tribuni for the nominative plural. There is no such thing as "Tribunis", which I pointed out earlier, and the EB team, clever fellows they were, already noticed before.

Miles, itis means just "soldier" in the strict sense, so it's normal that it would be in normal everyday use for any legionary. You're right about the plural.

Also, you need a good understanding of the flexing of nouns and their sense within the sentence if you ever hope to understand Latin at all.