Replacing front speakers, don't want to install non Bose/LS bracket and 6.5"

Replacing all speakers shortly and had a quick couple of questions. I have a 90 LS w/ Bose. In the front if I don't want to source a non LS bracket from the junkyard and use shallow mount 6.5" speakers, can I just install some smaller speakers in the bose box? What size fits in there? Are there any huge advantages to having 6.5" speakers up front over the smaller ones?

Also as far as the back goes it is my understanding that I can fit 6X9's or 6.5" speakers. Which is reccomended? Is there noticible quality differences between the two sizes? Thanks