If China feels that Singapore has alot of riches and strategically located and whatever, and if the Chinese here all want China to come protect it, then it is possible to happen. But as now, the Chinese here is not thinking they're from China, so it is ridiculous for China to say it is protecting its Chinese people, but you never know, with the PRC aplenty here, this may happen.
Finally, if China wants to take over Singapore for that reason and can justify it internationally, my guess is that nobody can stop China.
China is now the largest exporter and importer in the world. Frankly, China can not be sanctioned economically, because it can retaliate economically too. Besides, USA is beholden to China, since China holds 1/3 of USA's national debt.