I want to know which norms might have governed the spelling of some Russian names which were written down in Spanish around around 1750 - 1850. A number of formal standards exist today, such as ISO 9, but how would it have been done a couple of centuries ago?
Yes, although I'm afraid to say that then, as today, healthy chaos reigned. Transliteration conventions depend as much on other aspects of the context - the writer's familiarity with Russian, the intended audience, the keyboard or press available - as on the year.
So which conventions do correlate with year? These days, we can find the English convention 'sh' being used more, and also the letter 'k' in places where 'c' would work, and 'y' in 'Krylov'. We also see 'y' used in ways that correspond to Latin American pronunciations.
For example, Pushkin vs Puchkin:
Some aspects are still open questions - hence Aseev/Aseyev/Aseiev, Yershov/Ershov - and it is likely that the precise balance has shifted over time.
You can select from Rusos del siglo XVII and search variants of their names in the Google Books n-gram viewer for Spanish books from 1800 on.
If we go back a few centuries further, then 'x' had yet another value, and English was even less dominant.
Transliteration aside, in the past names were more frequently simply translated, hence Luis Adolfo Pedro de Sayn-Wittgenstein, José Stalin etc.