Looking at the 131's width = 85cm and recommended sails 6.5 - 10.5, you have a sail size dilemma. From a 7.5, your next sail up would likely be and 8.5. From there, somewhere between 9.2 and 10.0. Since the recommend top sail size is 10.5, a 10.0 may be pressing the upper limits.
Someone with experience on the 131 (133) may be able to offer some better advice. Personally, I have a 7.6, 8.4 and 9.2 and like the spread in sizes. None seem too close to the other in size. I use the 8.4 occasionally on my iS 111 and the 9.2 on my formula board (11.0 also).
The 131 comes with a 52 cm Drake fin. To maximize your light wind capability, you should go for an extra fin, around 60 cm if you move up to the 8.5 and 9.2 sails.
All this assumes that you frequently have light winds and want to get the most out of your gear. Hope this helps.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.