Whether you need a first-aider really depends on what the girls will be doing. Meetings and activities don't automatically require a first-aider to be present, but I do recommend that leaders and/or assistants have CPR and First Aid training because, with children, anything can happen at any time.
The guidelines that you absolutely must follow are those printed in Safety-Wise and the Policies and Procedures manual. Use the index to find the specific information about the safety guidelines that apply. If you have trouble finding something, ask a Service Unit or Council resource person. I'm always happy to help you.