The only reason I don't recommend carving forms is because you never know who you are dealing with or what their skill levels are. Not everyone can carve a form even though a gator is probably one of the easiest things you will ever carve. Because of that I do recommend the fill method.
I know Bubba already answered your "fill method" question but I will just expand on it a bit. Usually guys or gals will fill the feet with apoxie or clay up to the wrist/ankle of the leg and then fill a little of the leg at a time packing it tight using dowel rods of various sizes. The tail and rest of the body will be done the same way as well. I know a lot of new people get tired of us always saying, "buy the damn video", but one of the main reasons we say that is because the video is better than any written tutorial that we cold provide you with. You can always refer back and forth to all the videos and play them as you mount your critters. So having said that, next time one of us old grouch asses says buy the video take it with a grain of salt and when you do buy the video you can come back ans tell us how great our advise was because you love them!
I love to help people and honestly there are days where my phone seems to ring 100 times but in reality it only probably 20 to 30 times.
Good luck on your projects all you scaly, creepy crawly loving bass turds. LOL