That was a long post so I am going to hit the high spots ok :)
Rules of using work out equipment at the gym, you have shortness of breath or chest pain, you stop the activity and rest. If you feel ok, you can push yourself a little harder but never when you are having symptoms.
You posted "sinus arrhythmia" your EKG showed a sinus arrythmia, what was it?
Sinus arrhythmia is the variation in the spontaneous rate of the sinus node, resulting in variation of heart rate. Is this what your EKG showed.
If the Holter Monitor captures a SVT and you have sx I.E. your heartrate is 200 BPM please stop the exercise.
As far as treatment many people with SVT are controlled by meds alone. My best advice, stay in contact with your physician or find a good cardiologist.