They can but if the heavyweights are all supporting the inclusion, they will be at a risk of diplomatic isolation so have to vote yes.
Laos a heavyweight torpedoed a joint statement on the South China Sea
That is not entirely true, In 68' the Lankan economy was more advanced or atleast on par with Singapore, Infact LKY emulated many aspects of the Ceylonese economic policies at that time in rebuilding Singapore, Colombo port was a direct competition to Singapore as a hub, LKY and other Singaporean policy makers knew this, Unfortunately for Ceylon the Civil war took over and the country regressed
Sri Lanka couldn't submit an application because it was shot down by the Singaporean president, Remember this was at the formation of ASEAN before even the charter was made, Anyway as many mentioned before a Charter is not set in stone especially multilateral ones, Clauses can be changed owing to global and strategic necessities, I think currently Singapore has a more bilaterally friendly relationship with Colombo, Rest of ASEAn especially Thailand and Indonesia have a very strong relationship with SL, If they choose i dont think there is any hurdle to include Sri Lanka in to the organization although i dont see it happening in the near future
Btw if i may ask are you a Malay or a Chinese Malaysian ?
Comparing with Singapore in 1968 is unfair because Singapore got its independence in 1965