Every hotel I've ever been to in Tunisia (including at least one of the Merhaba group in Sousse) has insisted on no shorts for dinner. Remember it is a muslim country and you should respect their accepted standards. You don't, however, need to "dress" for dinner - smart casual will be fine.
As the currency of Tunisia is the dinar, I've always found that the best currency to use!
It's a while since I've been to Sousse, but I've always liked the town. Be careful in the Medina (this goes for any North African town); don't get dragged into "my cousin's carpet museum", which will be a rip-off hard-sell poor-quality carpet shop, and don't fall for the old "I'm a waiter in your hotel and I'll show you round" routine - you can see, I hope, where THAT one will lead! All Tunisians will understand French, if that's any help to you, and a polite "La, shokran" ("No, thank you") accompanied by a smile should stop you being bothered too much.
Taxis around Sousse are pretty cheap, but you need strong nerves; good excursions to go on are the Roman remains at Dougga and the 2-day Sahara trip. Tunis and El Djem (the Roman amphitheatre which was used in "Gladiator") are easily and cheaply reached by train - both worth a trip.
Enjoy!