Sai used it when Pica attacked him. Zoro's Haki is better than Pica's so he can use it.
No.
Sai did not use it. He only partially coated himself with regular Busoshoku Haki.
Just because your Haki is stronger, doesn't mean you're as skilful in using it. Similar to real life scenarios such as: Being a stronger weightlifter, to lift incredibly heavy weights they need good form. A weaker person with better form is therefore more skilful in the act, however they aren't as capable as the former.
Or, if you're too stupid to understand the last example:
A football (soccer) player like Ronaldo can drive his shots very powerfully. However, his freekick conversion rate is shockingly bad. Players like Calhanoglu have much better free kick conversion rates and less powerful shots.
There are many other dumbed down examples for you to understand should you fail to understand even these simple sporting examples which emphasise the difference between skill and strength.
Until it is shown in the Manga, a character cannot use Full Body Koka.
Full body Haki is a skill, not a power statement. FFS.