They are nippy, so they may bite little chunks out of the fins of your fish. It is hard to say whether they will do it or not, it is a personality issue, and that is related to other inhabitants, etc.
CARNUT,
Sounds like you are new to this hobby, and you've started out really well by asking questions here. I encourage you to keep asking questions, because you can not go out and just buy the tank, get a filter and get the puffer. It takes quite a while to set up a saltwater tank, and is an expensive hobby. The most important think is to be very patient, and read as much as possible. You should do some searches about setting up a new tank, filtration, etc. It will take time, but will save money if you do it right. It will also save fish!
A scooter blenny tends to have a diet that is based on having lots of good healthy live rock, and a small tank (29) is not the best choice for them. I am a bit worried about a cardinal and a Valentini in a smaller tank. But that is my opinion...