The -ist suffix comes from Greek. It can be Latinized as -ista. Words with that suffix are masculine. So you could possibly have computista (masculine), but that word would be a bit of a monster because it's a mixture of a Latin root and a Greek suffix. Such hybrids aren't unheard of, but they aren't very nice, and there's no real reason to make one here. You could perhaps use the following:
There's no neuter version of this. A computist is a person, not a thing. With a few special exceptions, neuter nouns don't denote people. Agent nouns like these aren't usually turned into diminutives, either. You'd just add an adjective meaning "small".