To the best of my knowledge, both Synechocystis sps. and Synechococcus sps. can naturally synthesize sucrose both under unstressed and stressed conditions. These cells may synthesize higher levels of sucrose unders stress. Sucrose unlike glucose, fructose, maltose etc. is a non-reducing sugar and hence most preferred as it will not interfere with general metabolism even if it is present in higher concentrations.

Regarding the questions of whether, you can select Synechocystis elongatus with sacB Sucrose-counter-selection, it will certainly depends on (i) the level of sucrose that is present naturally within their cells; (ii) potential of these cells to withstand osmotic shock; (iii) type of gene that you wish to introduce. If you wish to introduce a gene that will help in withstanding osmotic shock, then there may not be nay problem. However, I believe that it will have severe neagative impact on over all sucrose and related metabolic events. I shall prefer looking for an alternate selection system such as codA-choline selection system.