Teresa - Glad you are finding this site useful. We are not aware of many other forums where you can bounce your questions directly off the authors themselves!
Your street saviness will work in your favor and your lack of Spanish will work against you. In general you can expect minor harrassment as a single female traveler, but not much worse than that. There are a couple of beaches where you should not travel alone - the Corn Islands are now among the places you may run the risk of violence. Rather than go with a tour, if you want flexibility, just meet up with other travelers as you go from place to place. On the gringo trail there are lots of other travelers who will probably be headed to the same places you are. We mention in the book the hotels that are most popular with travelers; these are probably your best bet for meeting up with adventure companions.
Meanwhile, use your street saviness to your advantage - no coming home late at night, no flashy jewelry, be aware of your surroundings, don't flash money around, and similar. You are probably safer in Nicaragua, over all, than you would be in Detroit/Washington DC/NYC/Miami.
Also have a look at this thread:
http://www.gotonicaragua.com/forum/index.php/topic,38.0.html