However, not all plumbing contractors are the same. Just like my post for electricians when you hire a plumbing contractor to work on your big project make sure they are licensed and make sure the person they send to your home is licensed. Some contractors will send non-licensed plumbers to work on your project so they gain experience, which is fine, but look for a break in the price if this is the case, or make sure a licensed plumber is on the site as well.
I have no problems with someone learning, we all started somewhere, I just want the teacher there as well. Wisconsin law requires a Master Plumber be responsible for the work, the problem is there is a way to get around the law. A shop can hire a Master Plumber and use his license to pull permits and have non-licensed employees do the work, which, again, is fine as employees need to learn. However, this can be abused by some shops as they have employees do work they are not trained for or have no desire to learn. For example, you could have a multi-trade Plumbing and Electrical contractor where the plumber might be installing his own electrical under the Master's license working for the contractor. The guy might know what he is doing, and odds are he does, but you are paying to have a licensed electrician do the work, so why not ask for one.
I recommend you hire a contractor for one problem and a different contractor for the other problem to avoid these types of conflicts, or verify if separate people will show up.
The other insurance policy you'll want is to check if a plumbing permit is needed for the work you are having done, and then make sure you get an inspection done when the work is finished. The other advantage to doing this is the multi-trades contractor will be more likely to send the proper guys out for each portion when they know permits are needed. At a bare minimum the work will get inspected/checked to make sure it complies with code.
If you have a big plumbing project give us a call and we'll see if we can help. If not, we'll recommend a couple people to call.