Hello and Welcome Gordon M.
I will try to help, and I am sure others will too when/if possible...
This frame will be approx. 21 1/2 ' x 21 ' outside of the footprint of the posts.
That is rather large for a first frame...yet doable if you have solid basic woodworking skills now...
I have looked at the 19 x22 pavillion on the TF headquarters site and was wondering if I could alter the dimensions of those plans to make it work in my situation?
Modifying or developing your own designs without prior and solid timber framing skills can be a risky venture. I personally strongly recommend against it for most students just starting out, unless they have direct "hands on" guidance from a professional, and/or PE approval for the design.
Do most plans or drawings show details of the joints to be cut?
Yes, the better ones do have this, some also come with phone support, all good plans cost money. Usually starting at $5 to $10 a square foot for basic (not custom) frames...sometimes less for smaller project...seldom for as large as yours in my experience.
Is there a book or somewhere on the web that has more detailed drawings and procedures for cutting joints than the books I have?
The books referenced should be all that is really needed for a basic frame. Again, as a Timberwright that designs, builds and teaches, I can't really recommend to most folks to go beyond a small garden shed without prior experience, and/or hands on professional support.
Is there someone on this forum that i can hire to design a frame with detailed drawings of joints etc. for this frame?
Some members here have a solid background in timber frame design work and facilitation.
Would a hammer beam type pavilion truss be too hard to cut / assemble for a first time frame builder like myself?
I would not recommend it...but some have taken on such a style of frame with great success. HBT frames are not something in general I would recommend for a first timer nor for a pavilion...A Cruck Frame perhaps or something in the Asian modalities...It all depends on what styles of architecture you care for...