Let’s remember that the purpose of grammar is communication — not torturing language learners. In your example, the context (including the preposition “after”) makes the sequence of events so obvious that the exact tense doesn’t matter. In fact, we don’t even have to conjugate the verb at all — we could simply use a gerund:
After graduating from college, I started working.
Or a noun:
After college graduation, I started working.
It’s usually good style to use the shortest version that clearly makes your point. Just one thing — if a sentence begins in a subordinate clause or a long phrase of any sort, we need a comma after it. (See my examples above.) You don’t have to memorize lots of crazy comma rules — simply say the sentence out loud and notice if you pause. (If this doesn’t work, then you need to work on the non-verbal aspects of your spoken language.)