Classes are set in stone if you mean switching classes. However, I would consider allowing leeway for personal customization of characters for skills that are not covered by the stats. For your second character, I see either the Barbarian (and note this class should be seen as a Herakles/Ajax-like character) or Spearman. For the third, I was looking over the manual for Mazes and Minotaurs and there is a thief class. The only reason I did not include it because it had other special rules and I did not want to add more complicated material that would in the end be optional. I would really have to think about the first one.

For assigning stats. Let's see if I can explain this better. After I give you your rolls (I will give you five sets of 6 numbers so that you have options to choose from), you take the two highest numbers and assign them to the two attributes that are listed under the "primary attributes" section in the class section. The remaining four numbers, you get to choose where they go. Each value that can be assigned as has a modifier (the 'Y' value in the character sheet). Below the example in the attribute section is a range of numbers. To the right of each range is the corresponding modifier value. This modifier value is used to calculate the rest of the stats.

To be honest, I kind of regret showing the "Saving Rolls" (Athletic Prowess, Danger Evasion, etc.) because without a clear explanation, they can seem quite confusing. Basically, they are used during situations where you are avoiding danger, performing an athletic maneuver (climbing), exerting physical strength, etc. There is a roll to determine whether you succeed and these values are added to the die roll.

Please ask anymore questions if anything is still unclear.