OK, I follow what you want to do, but it depends on how you want to represent it. If you have "consulted with your experts" and they say that a given task with certain people will take a particular amount of time, lock them in to that. In MS Project, if you go into the Task Summary form (double click on a task) and click the advanced tab, turn off 'effort driven'. This will stop the task duration being affected by the number of people and their time allocation.
You can manually adjust the allocation of a person to a task in hours or percent also, on a day to day basis. And, you can even adjust a person's cost table on a task by task basis, according to payment structures. ( i can elaborate, but let's get back to the project)
If you can fix the duration of a task (and hold the workers to the duration, (ha!)) allocate all the people you need to the task. This is more easily done in the task properties window. You can create coarse allocation, say 50% overall, to a particular person for that task.
Now the tricky bit begins.
With the Gantt displayed, use the vertical window splitter or choose Window>Split; click into the lower section and choose Resource Usage from the sidebar or View>Resource Usage. By default, you should be seeing the resource allocation for a SELECTED task, if you don't see anything, click on a task which has resources allocated.
You can manually allocate hours per person to avoid overallocation, or use percentages. Percentages dont show up by default so right click on the resource allocation view and choose Detail Styles. From the list, put in Percent Allocation across to the right window, leave in Work and click OK.
You can only change the hours allocated, but the percentages show you over allocation pretty quickly.
For any resource on a task, the work contour can be adjusted; if you know that a task will need expert (expensive) involvement toward the end, you can adjust that weighting automatically. Double click on the Task heading below a resource for Assignment Information; change the Work Contour as required and the allocation of hours will be adjusted.
Any task which has been manually edited will have a blue graph with a pencil in the information column.

The main thing is to get your baseline right for the client. Once you've set the times and everything in place, save a copy and then see what Tools>Level Resources buggers up. It might push tasks waaaay out, but if everything is cool, there shouldn't be too much change. Then do Tools>Tracking>Save Baseline. This then lets you compare other data with a known starting point. If you want more info on optimistic and pessimistic allocation, let me know.
The next impressive part is tracking actual costs/time. But that might be too much for now.

I hope this gives you a hand in your project.