I've been in tech recruiting for 8 years. Mix of inhouse and contingency search. I'm not sure what the IT recruiting market is like in the Netherlands so my answers are from a US perspective.
1) 4 months is doable as long as you can generate enough viable job orders for the recruiter to work on. 2)Depending on the niche 2 hires per month is doable. 3) Base + commission is a standard structure for agency recruiters. You can add a progressive tiered commission structure to incentivize the recruiter to go above the monthly goal (eg. 0-$40k fees a month is 10%, 41-80k 15%, etc) 4) 6mo contract gigs are pretty standard. Will the contractor stay is a question of the quality of the role and the pay. 5) For direct hire 20% of the yearly salary is standard. I've seen as high as 40% for highly impacted markets and skills. Others will take as low as 15%