I have owned 4 quads, the spider type such as the Gem Quad, and Lightning Bolt, also Cubex boom type and homebrew quads of each type.
I just retired my 3 element 5 band quad made from Cubex parts which had been in service with a Glen Martin Hazer. and Rohn 50 ft tower since 1989. I would offer the following suggestions to you:
1. Before buying a spider type quad such as the Gemquad, make sure it will work with the hazer for clearance to the tower. A boom type 2 element such as the Cubex will have an 8 ft boom which works with the hazer. I see that you have already bought the Glen Martin Hazer, good move.
2. Use a remote 5 band antenna switch with one feed line going to your tower and the switch, then use individual feed lines to each wire element of the quad. This way you will notice no interaction between bands although some is there it won't bother your signal so that you would know it. RG 8X is what I have used for years with good results.
3. Tie dacron sun resistant ropes to the hazer itself for guys on the tower. Once you have them adjusted, they simply come down with the hazer and nothing further is required. One set is fine for the 50 ft Rohn based on my experience. You will need to keep them fresh say replace after 10-12 years.
4. Take a look at the individual coax homebrew choke baluns used on each wire element by John Tait, EI7BA.
I found his method of feeding to be the best I had used over the years for long term service.http://www.iol.ie/~bravo/Cubical%20Quad.htm
I modified my Hazer recently and have replaced my 3 element cubical quad with a 2 element hex beam broadband design of G3TXQ. After owning and using the little 25 lb hex beam, I must say it has not been a let down after owning quads .
I have a photo of the hex with the hazer on QRZ.com
I recently gave my old quad well done and decided not to replace a spreader that I broke by being careless. Quads are great antennas .I have also owned trap yagis by Mosely, Hy Gain and others no longer with us. None performed as well as the 3 element quad at 50 ft.