The definition of manufacture is defined under section 2(f) of Central Excise Act.
Basically a process amounts to manufacture only when:
1. A new and distinct product having distinctive name, character or use comes into existence. 2. It should be fit for commodity use and has a commercial market.
Merely by putting large amount of goods into small amount of packings (as it is in your case) does not amount to manufacture. But if you are mixing the products in such a way that a new set of product comes into existence which has an entirely different market identity then it might amount to manufacture.
Answer to your second query is that all the manufacturers having a turnover of less than 1.5 crores in a financial year fall under SSI exemption. You do not need to take excise registration within this limit but the moment your turnover crosses Rs. 90 Lakhs in a financial year you need to inform the excise department.
Another thing I would like to tell you is that what ever process you are doing that amounts to manufacture or not, the onus is on the department to prove that. In general practice they might not reach you till the time you are within SSI limit. But the moment you will cross that limit this issue of manufacture has to be sorted out with the department at the initial stage only. Otherwise it will always be a problem in your future working.