While Metoprolol is techincally a "blood pressure medicine", it has very minimal effect on blood pressure and mostly affects heart rate. Most people tolerate the medication very well. If you are nervous about its effects, it is not unreasonable to start by taking 1/2 a pill, as long as you know that aborting or controlling your SVT may take more than that. You could start with 1/2 a pill, and if that doesn't work, then move up to a whole 25mg pill.
I take metoprolol at bedtime. My doctor suggested this because I found this medicine made me dopey. For instance, I couldn't recognize people I'd known for years. Taking it at bedtime helps with the side effects, in fact for me it acts like a sleeping pill. I sleep six solid hours when I take it.