Raspberry Pi is the most related community to Adafruit on stackoverflow, I am wondering if I can ask questions about Adafruit here?
That would depend on the question you are asking. If you were asking about a Pi peripheral from Adafruit, that would be on-topic:
Force Minecraft-pi to use PiTFT, not HDMI (Adafruit PiTFT)
Adafruit 16x2 LCD not displaying properly (Adafruit 16x2 LCD)
Problems with following an Adafruit guide would generally be on-topic as long as the task was something specific to the Pi. What is considered "specific" to the Pi is often unclear, but generally if you're configuring your Pi, connecting hardware to it, etc, you should be OK to post here.
Questions about setting up an electronic circuit may not be on-topic if you're not specifically interfacing it with the Pi. If you had a reason to suspect that the Pi is part of the problem, then it would be on-topic here, though.
Obviously, questions about Adafruit's Arduino offerings would be off-topic here. The Arduino site may welcome those questions though.
Asking "What should I buy from Adafruit?" would fall under the explicit ban on "specific purchasing recommendations" as it stands. I'll quote the guidance in the help center as it is:
[...] Please note that the following is off topic
Asking for specific purchasing recommendations: This refers to questions such as, "What power supply should I buy?" (= off-topic). However, if you have a specific product and you want to know if it is suitable for a specific purpose ("Is this power supply okay for a Pi Zero W?"), that's fine. Asking what to look for in a kind of product is also fine ("What is the minimum amperage power supply I can use with the Pi 3?").
All in all, your question will generally be on-topic if it's about Adafruit hardware/software if you're doing something with the Pi and suspect the problem lies with the Pi. If that turns out not to be the case, others will guide you to the correct site in order for you to get help.