Fleet or other disposable small volume enemas may be fine, or even a good idea if you are just starting out. But they are different than using a bag in several ways.
Most obviously of course, is appearance - there is more to an enema/douche combo bag than there is to a disposable. When you start using a bag, you will find that having that hose length is beneficial if you like to move around or get in other positions.. In fact, you should be able to position yourself in more ways than you could if by trying to hold a Fleet bottle in another position besides the knee-chest or left-side as they show on the box.
Either the enema tube or the fountain douche nozzle will work well for taking an enema; most folks seem to prefer the douche nozzle. I think that I get a slightly slower fill with the douche nozzle, and course you can control the flow with the hose clamp (or by just kinking the hose) while using either of the attachments. How high and how you hang your bag plays a factor in flow as well. Think again of the benefit of the hose length.
The Fleet enemas pretty much state on the box that they do not clean the entire colon, just the lower rectal path. If ya can't poop and you're "blocked" right there in that area, a Fleet usually works pretty well. A 2qt. enema from an enema bag will clean out that area, and beyond. Not to be gross, but you will probably be surprised at how much more comes outta ya.
It takes a little longer to fill up from an enema bag than it does to get even a slow squeeze from a Fleet bottle. You will probably find that slower fill to be more pleasant; you don't need to have anything harsh in your bag like they put in the Fleets. Use plain water with a little baking soda in it, or just plain water for a nice mild fill your first times.
If you start feeling too full clamp the hose and stop the flow, pull it out, and get on the toilet. You may be only able to manage a third or less of the bag your first and maybe first few times. If your a small person you may never get to the point where you can take an entire bag all at once (although some smaller people can, at least occasionally). It doesn't matter though - do it how you like to do it. If you have liked taking Fleets, you will probably like using an enema bag too.. Time to graduate! I am sure you will find your own special method for using your enema bag in a way that will please (and pleasure) yourself the most.
I still take a Fleet enema every so often on a rare occasion. I even had a friend give me one a while back before he gave me several bag enemas. I had brought a Fleet Enema Extra with me when I visited him, but wished now that I had brought a twin pack of Fleet Naturals instead and had him squeeze both of those up my rear. I am really not that fond of the original Fleet formula anymore.
I believe that using a bag is probably safer for more regular than if you are using the regular formula Fleet enemas. Remember: too much enema usage of any kind could lead to a dependency. By the way, what kind of Fleet enemas are you using?