It depends on the transformer as to whether it can be a straight swap. Some have minimum power demand requirements... so you might see a transformer rated 20-60w. Less than 20w and you're not guaranteed to get good quality output regulation (or even any output at all).
As for fire rating... it's not as simple as which floor it's on. If the house provides a single unit of occupancy for a single group of people, then none of the internal ceilings/walls are considered to be fire barriers. If the same property is split into two flats, ground floor and first floor, then the ceiling between the ground/first floors needs to be a fire barrier because whilst it is a single building, it's two distinct properties. I could be wrong with this and I'm sure someone will chip in if I am. The best plan for this is check building regulations, but I would ordinarily try and fit fire rated units where ever possible.
Why do some downlighters need a transformer and some just a lamp... because they have different supply voltages. GU10 LEDs have voltage regulators built into them and are supplied by 240v, MR16s don't and as such require a transformer to supply the 12v they need. This is somewhat overly simplified because most LED lamps have some form of regulation built in.
