Don't worry to much about mycorrhizae if you are doing a hydro style grow. Mycorrhizae is for plants growing in pots that will be absorbing all the water through the roots until the plant is dry. Mycorrhizae make nutrients more available by breaking them down and feeding them to the roots, at the same time they are making water more available in the same process. If you are using a program like H&G in hydroponics, you will see a minimal difference from mycorrhizae, it may or may not be worth the extra money. I have ran the hydro formula in DWC with mycorrhizae, and did not see enough of a difference to continue on with it unless it is a micro grow for personal use, and you are trying to get the absolute most out of the few plants you have. I did have better results with my DWC mycorrhizae grow, but there was not a big enough sample to make any conclusions. From most of my research, however, there isn't a huge benefit to hydroponics and endo mycorrhizae.
What you will be doing for hydroponics is a little different than DWC, but the roots are saturated in a media for most of the day so it isn't much different. Phosphorus is available to plants at 25 ppm, so most fertilizers containing P will have at least that. Now, mycorrhizae won't be totally useless if you add synthetic P, I have done it and gotten visible results from mycorrhizae. But it is much more effective in an organic source of P.
Since you are doing it hydro style you could totally get away with 2 gal, but 3 gal would give you some plants with beefier stems. We like beefy stems.
You have a bitching sweet TDS meter, but it won't tell you how much P is in something. You will have to call House & Garden up and ask. You will probably get Rob. He is a real busy guy but he is very informative and a very cool guy if you ever get a chance to kick it with him. So give him a break if he takes a little while to get back to you, he does the same amount of work as a 9 man team.