OK. with all the Drama about number of mercs and game rules, and stuff.
Scenario stopwatch. is a stopwatch match with strict rules of how much mercs are involved. and class-objects. yes, you heard me. 
Lets take OLD Waterloo map as an example.
it had:
3 C4 objs (first door and two containers)
1 Hack obj (that sweet sweet balcony object)
2 Build side-objs (lol Pumps)
that means Attacking team HAS to have
1 soldier min and 2 soldiers max - IMPORTANT - because Only soldiers can plant c4 in this mod.
1 enineer minimum - because only Engies can build side objects
1 Covert-Ops - because only covert can Hack.
bunch of other mercs
Defendors Need
1 min and 2 max Engies for Defuse.
1 min COvert to JAM hacking process.
bunch of other mercs
so the map itself dictates which mercs do you need to fill the minimum nessessary squad.