Project 2013 Newbie Questions

[Steve.]

Hi all firstly would like to say what a great site you have here, a trove of information

I have just started learning project yesterday and I'm currently working my way through the step by step guide. I have however run into a slight problem which is,

When I assign an addition resource to task (depending who I have assigned) the duration gets longer. The only changes I have made are working hours which are now 07:00 to 12:30 and 13:00 to 16:00 and I have also change 2 resources(these have 1 day a week set as a non working day but still have 100% )

Why does project add time instead of reducing time ?

How do you guys set up time schedules ? do you start with estimating the time needed based on a single person and have all tasks set as "effort driven" ?

any help would be great and thanks in advance.

EDIT: I guess what I want to do in a round about way is, I have 3 apprentice one goes college Monday the other is Tuesday and the last is Wednesday if the task is set for 3 days, mon-wed and I need 1 with a engineer everyday(doesn't matter which one) how does Project handle this as atm it offers me to reschedule the task around them which I do not want to do.

Hopefully this makes a little more sense.