You absolutely can choose what methods of hair extensions you want to become certified in. Whether its one method or all the methods.
There are all different types of hair extensions courses to take. Some courses only teach one method and then you'll usually end up using that brands hair as well. Some courses teach a few methods and some, they teach all.
Although you are able to become certified in one technique in a matter of hours, are you really learning everything you need to take your career and/or business a step further?
The most common methods of hair extensions are clip-in, tape-in, sew-in, and fusion.
Not all of your clients are going to be fit for the same method and same brand of hair. So, the more methods you are certified in, the more you can offer your clients.
Your clients will appreciate you taking the time to explain to them what method you believe they are most fit for and why.
At the end of the day... Yes it is your decisions and can pick and choose what methods od hair extensions you would like the offer your clients. Just keep in mind how much further you could grow your business if you offered more!