Keeping anemones and anemone-fishes is wonderful. It can be worth waiting until you are sure the tank is properly settled.
Nems do tend to go into shock if the water doesn't seem natural to them, you can try to flush the water column with lots of small live foods (like rotifers, Copepods etc.) while you are introducing the Nem, so its instincts feel the location is good.
I keep a macrodactyla in a 60l and haven't had any issues with size compatibility but avoid mixing anemones with corals. They are different types of stingers and tend to suppress eachother by letting out toxins in the water, which is best controlled by overpowered skimmers.
Also the macro is a sand dweller and it does not split so you won't have to remove clones from your liverock as might happen with bubbletips.
Since the fishes probably already have a favourite area in the tank, you should try to get the anemone to settle at that spot. It can be alittle work to get the Nem into the spot that you want, just keep working with the current and the rockwork until the Nem and the fishes accept the same spot, the fishes won't leave a favourite spot easily.
If you don't have a hospital or isolation tank, be prepared to remove and lose the Nem, they keep moving while they die but they rot incredibly fast if they do, and you don't want to lose your fishes to a rotting anemone.