RBTAs can grow very quickly. I've had mine for about 2 months, and it's doubled in size. So even if you get a small one, it will grow quickly, and can overtake a tank quicker than you may think. Also, the nem should be at least twice the size of the largest clown to make sure the clowns don't "overlove" the nem.
Make sure all powerheads inside the tank are either removed, or covered with some form of plastic to make sure it doesn't suck the nem into it. You also need to be aware that they will move around to find a spot that they really like, so be ready to move any corals that may be too close to it.
Also make sure your tank has been running stable for many months. They can be very finicky to bad water.
And last but not least, if your clownfish don't immediately take to the nem, don't fret. It can take awhile, and you could always try the photo method.
Otherwise if you're willing to try it and know the risks involved, the symbiotic relationship between clownfish and an anemone is one of the most amazing parts of reefkeeping.