Yes. The drain plug is an 8mm square key. The threads on plug and casing should be clean and plug refitted with thread lock sealant.
Fiat state 2.7 litres refill quantity for the gearbox, but I've been told that newer boxes are delivered with 2.9 litres refill specified on them. The Citroen Relay manual also states 2.9 litres refill for 6-speed boxes, and 2.7 litres for 5-speed.
Fiat fill them with Tutela Experya SAE 75W80 transmission oil, but following a chat to a dealer mechanic I've refilled mine with Tutela Matryx 75W85 which is a higher spec fully synth oil. It's much smoother and has lost the slight notchiness in the gear change that was previously sometimes apparent.
Citroen recommend Esso EZL 848 or Total Gear 8, both again 75W80 in the very same gearbox, so I would say the correct viscocity and quality matter more than the brand of oil. It's important to use a GL4 spec oil, not GL4+ or GL5.