What's the difference between atrial flutter and svt- and can they happen at the same time?

Similar-different. Both originate with the "pace-maker" in the right atrium. In svt, it leads to a sinus rhythm over 100-140 beats per min (bpm). Atrial flutter is a specific type of svt that causes (by definition) an atrial rate of around 300 bpm. Depending on the electrical conduction of that heart, it will lead to a ventricular rate of some multiple of 300 (150, 100, 75, etc).
More specific. Svt stands for supraventricular tachycardia, or a fast heartbeat coming from the top part of the heart. There are many fast rhythms coming from the top of the heart: atrial tachycardia, atrial fibrillation, junctional tachycardia, and psvt (paroxyxmal svt)... Atrial flutter is one of these rhythms, and is characterized by a regular, rapid rate of the atrium typically between 290 and 310 bpm.