I would choose the board(s) based on the sails and Fins which you use with them. Also considering the waterstate and conditions you planning to surf is helpfull. For example I know a lot of windsurfers that like a monoconcave bottomshape for lightwind up to mediumwind in flat- mild choppy conditions, Starboard and PD offer(ed) a monoconcave board.
Chosing boards from different brands can be interesting, if you know what you want. Testdays are great to decide. Or consult local windsurfers (sponsored) with gear you would like.
However for most I believe 'logical' combinations would be the safest bet for fun and performance (fanatic-duotone, Starboard-severne, point7-av, etc. ). Other combinations might work great or might work (far) below average performance, you will find out. For example, Who knows if a iSonic works great with the current Ga-sails?
Another strategy is buying boards without a related sailbrand (futurefly, i99, etc.). Often these designs are a bit more neutral because teamriders use different boards. Same with sails without a related boardbrand (loft sails, s2 maui, er.), these are often more neatral designer.
For example, I was starboard fan for a long time, however the latest iSonics (I believe Iachino his sailing style influence) are not what I like. The fat ass of these iSonics combined with thin nose and short lenght is special to ride, but not better or fun for me personally. I did like the iSonics during Albeau/Dunkerbeck days a lot. Luckily I bought one and starboard sell good. Then I looked for boards which looked more like the older iSonics and ended up with Tribal boards now.