The new type of worker, known as colonus?

Question

replaced slaves on large estates , was engaged in manufacturing , was a member of a legion , organized trade association or collegia?

Answer 1

From Roman Colonus to Medieval Serf
The small farms of republican Rome were of 2-3 acres each. Grains, vegetables and fruits, including figs, olives and grapes, all that a family would need, were grown in fields fertilized by the cattle that pulled the plowshare, and rotated to prevent exhaustion of the soil. (These austere and very serious people also set aside a little space to grow flowers. And they were very fond of apples.)

But war put an end to this independent way of life. Whereas the wheat plant, for example, is an annual, grape vines and fruit trees, once planted, require many years to become productive, and olive trees, as the saying goes, a man plants not for his children but for his grandchildren, so long do these trees require to reach maturity. Because of this, farms gone to seed due to their neglect by absent soldiers and destruction done by roving armies and brigands, and ever-increasing taxes, forced small farmers to sell their land to wealthy patricians (patroni). The patroni combined these lands into "broad farms" (latifundia) dedicated to pasturing cattle and to raising cash crops tended by conquered peoples who were now slaves.

The source for most of the above two paragraphs was Will Durant's Caesar and Christ (New York, 1944) iv, 5. Some other matters of fact below are also based on that author's work.
The small farmers of Rome had been called coloni, a word that simply meant "farmers" or "cultivators of the soil". But in the new way of life, coloni took on the meaning of "tenant farmers", i.e. farmers who work land owned by another and pay rent either in shares of produce or in money. When, eventually, slaves became too expensive to keep, they were set free -- to also become tenant farmers.

Although these Roman tenant farmers may seem to have been more fortunate than their descendents, since they had to surrender only one-tenth (as opposed to one-half) of their produce or to pay a small money rent to the landlord, the result was the same. The inevitable crop failures -- caused by hail, drought or flood -- that hit all small farmers and the ever-increasing taxes forced the coloni into a cycle of never-ending debt.

When debt or despair became overwhelming, the temptation for these landless peasants was to abandon the Italian countryside. Fearing that no one would be left to work the land, in the 4th Century A.D. the Roman Empire put an end to this flight: tenants were bound to the land until all debt was paid off. But the debt could never be paid off, and tenant farmers became serfs. In this way a pattern of rural poverty was established in southern Italy that would endure for over sixteen centuries.