Let me start with some examples:
Leningrad / Ленинград (today Saint Petersburg / Санкт-Петербург)
Leninsk / Ленинск
Stalingrad / Сталингрaд (today Volgograd / Волгогрaд)
Gorky / Горький (today Nizhniy Novgorod / Нижний Новгород)
So apparently, if we are to name a city after a person, we have three choices: add the suffix -grad, add the suffix -insk (possibly with truncating a few letters of the original name, such as Leninsk instead of Lenininsk), or not modifying the name at all, such as Gorky.
My question is, what are the rules for this, if any? Does it depend on what letter the person's name ends with?