first promotion is guaranteed unless you did some major fck ups on your job.
The reason why you see 1-2 years is due to the promotion cycle and joining date.
e.g. if your ministry’s promotion takes place in July and you join in December, it will take you 19 months to reach your first promotion window.

SIM/SIT/local U doesn’t matter after you enter. It’s scholar vs non-scholar. i.e SIT scholar > NUS farmer.