HI,
Supplementary Allowances - Its dends on the salary. you should first covreagre the major component, then if any balance amount is due then it can be put as supplementary allowance.
Merit pay - Its a back up amount, which will be deducted from employee's salary & can be paid quarterly, halhyearly, yearly or monthly. It is deducted for purpose the, employee can not leave the job soon.