How safe is it to buy the FFG skirmish maps? Will they rotate out of tournaments?

They generally rotate out the oldest map every 3 months, so don't buy training ground if you're playing competitively as it's now out of rotation.

The next map to drop is likely to be Nelvaanian warzone in a couple of months

If you're just starting to buy them I'd buy the coruscant map and the ISB one (when available) then wait for the next map to get your money's worth