I have done triathlons with my functioning j-pouch, did an iron man last year. Hydration and fueling are important, need to get it right with your training. Do not experiment on race day.
Biggest issue as you point out is the swim with an iliostomy and bag. I presume you are doing a sprint or olympic length tri which will take on the order of 2 to 3 hours. For the swim, you will need to secure the bag to your body in some way. If you are wearing a wet suit, the neoprene will keep it in place but could be messy getting the wet suit off after the swim. If you are doing a pool swim, ?lycra top and swim suit may help keep it from flopping around. The standard tri shorts and lycra top uniforms may work. Experimenting during training is how you will sort out all of this. You will also need to muddle out when you need to empty your bag (probably you should plan on it in transition between bike and run, more easily done as a planned thing in transition area since you can stow ostomy stuff in your transition bag).
Do mock up transitions between swim and bike and then between bike and run so you will have an idea as to how things will be on race day. Don't beat yourself up on transition times.
You are to be commended to consider a triathlon with an iliostomy!