You're right that sequences and series don't show up all that much on the ACT - as our complete guide to sequences on ACT Math points out, there is at most one sequence question per test. In addition, many (though not all) ACT sequence questions can be answered without formulas, so if you have trouble memorizing formulas (or worse, tend to memorize formulas incorrectly), you might want to steer clear of formula memorization and practice solving these kinds of questions longhand.
Probability questions also appear relatively infrequently on the ACT - only 20-25% of ACT Math covers pre-algebraic topics, and probability is just one of twelve different subtopics in that area, appearing approximately once per test. And as with sequence questions, probability questions can often (although not always) be answered by plugging in answer choices or plugging in numbers.
If you're worried that you won't be able to solve these kinds of questions without formulas (or that it'll take too much time), then there are a variety of different methods you can use to memorize formulas. As stated in our article on how to use ACT Math formulas effectively, different techniques work better for different people - visual learners do better with flashcards,
movement/kinaesthetic learners do better by practicing drawing and writing out formulas, and auditory learners learn better by drilling formulas aloud. I personally find that a combination of all three of these techniques is helpful, sometimes with an added bonus of creating a song that goes along with the formula/information, so you should experiment and see which of these methods works for you.
Once you've memorized the formulas, be sure to practice using them in actual problems (both to help you remember and to help you learn how to put them into use - otherwise, there's no point in memorizing them). And if you haven't already, take a look at our ultimate guide to ACT Math, the 31 Critical ACT Math formulas you must know, and our popular guide to getting a 36 on the ACT Math section.