Hi James,
When I figure mirrors I use a program called Tex, available here:
http://home.att.net/~mikel/
It is a DOS program but very easy to use. There are others which may be better but I haven't used them and they look more technical to me.
I haven't really explored building refractors so can't help you there. If you want info on cassegrains then have a look at the second edition of Texereau's book 'How to make a Telescope'. The nasmyth is just a cassegrain with a flat mirror built in to the tube to bring the eyepiece to a comfortable viewing position. These sites might give you some ideas:
http://www.loptics.com/ATM/telescope...12in_cass.html
http://tlepage.home.mindspring.com/12_DK_telescope.html
Cassegrains are a lot more work - I have the optics for one nearly completed. I would suggest not attempting one until you have made a few newtonians.
Hope that helps,
Rod.