Food: For anyone with food allergies take your snacks/essentials with you. There is one supermarket you visit as you leave Tana and this is the last one you will see for the rest of the trip until you return to Tana, so stock up on anything you require for picnic lunches and snacks (I found rice cakes but not much for gluten free/dairy free diets). Snacks that you can buy en route tend to be biscuits, sometimes get fruit/cheese/yoghurts/chocolate. Breakfast is limited (breads/cakes/fruit) but being GF I often got omelettes or rice.
Footwear/walking gear: unless you have ankle problems, then good approach trainers are suitable for the trekking, if you prefer them to walking boots (I'd taken both). The rocks are quite grippy. There wasn't any muddy/boggy ground (although you come across about every other surface). Gaiters are useful to keep the dust out. If you ordinarily use walking poles then you will definitely want to take them (to both Andringitra and Isalo treks) as there are large, uneven, rocky steps.
Weather: I went in September. All the previous reviews mention the heat which we noticed in Isalo and the closer to the coast we got, however we weren't expecting a cold, rainy walking day in Zafimaniry village (approx 14 degrees). Also the first night camping in Andringitra was close to zero, so pack for all weathers.
Currency: GBP can easily be exchanged at the airport for Ariary (as can EUR or USD). You can change Ariary back to GBP/EUR/USD on your return to the airport but you need to have a minimum of GBP50/EUR50/USD50 (at the time we went). So either spend it, or pool the currency as a group if you have less than the minimum left. Note, once in the departures bit of the airport they don't (officially) take Ariary! They only take GBP/EUR/USD. In the rest of the country you can only spend Ariary. There are cash machines in Tulear.
Mosquito nets: all except one of the hotels (which needed mosquito nets) had them (not all towns need them). So if you have a mosquito net then you might want to take it just in case.
Language: if you have ever learnt French it will be very useful. Ask your Tour Leader to teach you some Malagasy words - very helpful.