A 6" is quite doable for the firsttimer. The thing to remember when doing the grinding is that the glass is flexing a little bit so you need to keep from putting too much force on it and to make sure that the "walk around the barrel" is done at a good pace. Spheres are done by making sure that the strokes are in all directions and no two strokes are the same way for either the tool or the mirror. If you stop by my website http://webu.wigloo.com/bobmay for an article on using plaster tools in the grinding of a mirror.
An 8" mirror will be a little bit more iffy and I would get the 6" done first and then try a 10" (20:1 thickness ratio) for the primary scope after you have gotten some thin mirror experience under your belt.
Also, when you are supporting the mirror on the table, use some outdoor carpet (the plastic stuff) as support and check your astigmitism when you have the fine grinding done.
It would also be better if you do the testing of the mirror on it`s cell and at a vertical angle so that the mirror is being supported on the backside rather than the sitting on edge that is normally done on the Foucault Test.
Ask if you have any problems.