I have ridden a rollercoaster and now I am dizzy.
The only things wrong with your example sentence is that roller coaster is two words, and there should be a comma after them. This is to separate the two independent clauses. Otherwise, the grammar is fine.
Contextually, most folks would connect the two clauses as cause and effect. That is not the best way to word it, though. Potentially the only time you have ever ridden a roller coaster was 15 years ago, and you found yourself sidetracked mid-sentence (by your dizziness, perhaps). To force the interpretation you want, it would be better to say something like:
I just rode the roller coaster, and now I am dizzy.
Since it seems like you are interested in when you can use conditional and what effect it has on your language, I will do my best to use your examples to try to explain. (This will be long.) As I mentioned in my comments, there is nothing wrong with the tenses the way you originally wrote them, so my examples are just of different possibilities and how they affect the meaning. Here is a possible rewrite of your first one:
Friend: A beautiful girl is walking toward me.
Me: Would you talk to her?
Friend: Yes, I would talk to her, and we would hang out and have a good time, and then we would go to a cafe.
All of the places I used "would" here don't make it more polite. You could say they make it more uncertain, though. The way you wrote it is more interesting. In your example, you are agreeing to sort of "live" in the story and imagine it's true. In the conditional, you are just talking about a possibility. So even though you both know that it's imaginary with both these types of language, yours is like you are both agreeing to pretend it's true for a while. Does that make sense?
Your entire second example could be rewritten in the past subjunctive and simple past if you added the word "imagine" at the beginning.
Imagine you were walking down the street and met a beautiful woman. You chatted and had a good time. It seemed like she was the one for you. Would you ask for her number?
The effect here is similar. It makes it less like a story you are agreeing to pretend is true and more like an imaginary scenario you are describing just to find out what your friend would do in that situation. You are keeping the awareness that it's imaginary in the forefront.
The more interesting thing here is that you could leave it in the present tense and still change the final question.
You are walking down the street and meet a beautiful woman. You chat and have a good time. It seems like she is the one for you. Would you ask for her number?
If you say "would you ask for her number" instead of "will you ask for her number," it makes it seem more like you want an answer to the question. That's because it breaks out of the indicative mood that the story is in, which breaks the narrative. You've been imagining the story together. Now, you are breaking out of the story, and you really want to know what your friend would do.
If you say "Will you ask for her number," it could still be part of the story. You might be planning to continue and answer the question yourself. "Will you ask for her number? Yes, you will. Will she give it to you? Yes, she will." This would be even more likely if you said, "Do you ask for her number," because then it would still be in both the same mood and the same tense.
Your last example could all be changed to past subjunctive and simple past, too. That has the same effect as changing your second example that way, so let's just talk about the final "will" in that example. That is one place here where you could use will, would, can, or could!
. . . wipe out anything that will stop them.
Using will here implies that if the aliens don't wipe these things out first, then they will certainly stop the aliens. (The things both want to stop them and can.)
. . . wipe out anything that would stop them.
Using would implies that the things will stop the aliens if they can, but they might or might not be able to, even if the aliens don't wipe them out. (The things want to stop the aliens, but we don't know if they can.)
. . . wipe out anything that can stop them.
Using can implies they are wiping out anything that is definitely able to stop them, whether or not those things have any desire to stop them. (The things can stop the aliens, but we don't know if they want to.)
. . . wiping out anything that could stop them.
Using could makes it a little less certain that the things they are wiping out are actually capable of stopping them. (The things probably can stop the aliens and they might want to, but we aren't completely certain about either one.) This might be the best choice. It makes the aliens really evil. They're wiping out every potential threat, whether or not it's really very threatening.
I'm sorry this was so long, and I know the examples are kind of specific and might be hard to generalize, but I hope it helps!
Best Answer
In conversations, storytelling and discussions when we see something happening just now, at the moment of speaking we can use the Present Simple or the Present Continuous, and the Past Simple.
Both your examples are correct.
Here are more examples: