Tasks and age of Military Tribunes during the Late Republic and Principate

[Flavius Inismeus]

having embarked upon the senatorial cursus honorum, how could he serve as an equestrian tribune?

Ah yes, you're surely right. I was confused by Pliny becoming sevir equitum romanorum after his tribunate, but it seems that other holders of this honorary office were bound for entrance to the senate. So although he's nowhere actually described as laticlavius (as far as I know) we can probably assume this was his position.

Thank you for the explanation, now I see. Serving senators could and did take up the sevirate too - there's a prosopography in L'ordre équestre sous les Julio-Claudiens (1988) by Ségolène Demougin, pp. 226-240, that has a few ex-pretors, as well as tribunicians and quaestorians. 

https://www.persee.fr/doc/efr_0000-0000_1988_ths_108_1