Its good to expand the toolbox- in your case incorporating flash.

You would want to consider a flash meter if you dont already have one. You can delve deep into small flash lighting- You can get a lot of input, especially on new product, modifiers, lighting diagrams etc at the blog. Even though it is geared for more modern digi users the principle and techniques yield the same results.

As far as flash models and brands- the vivitars are a good place to start for manual off camera units. The Nikon and Canons are great, too, but are loaded with features that you wont be able to utilize on your sq.

All the best-