Having worked in the industry, I have seen FICO scores over 800 with very little credit depth to them...maybe two items, that's all.
I also have seen credit reports 4 pages long, yet again they had good credit scores.
The key to a decent credit score is a combination of things, but the most important are:
1. Pay all bills on time, all the time.
2. Don't run up debt quickly. The scoring algorithms realize when someone is running up debt quickly (and it does not really matter if you are using one or several credit sources to do that)
3. Don't over utilize your limits.
4. Don't sign up for small fry credit cards just to get some discount or spiff. Stick to the main players. (that is not neccessarily going to lower your score, but for many types of credit a set of human eyeballs still look at the report..and they don't like to see low grade credit cards (bottom feeding discount stores, jewellery stores and the like)