in general, round speakers are better than any oval speaker. so i'd definately suggest getting that. for the adapter plate you should make it out of a dense wood that is about 1/2 thick so it doens't vibrate and also fixes any depth problems. to get an idea of what i'm talking about look at someone elses car that i saw. http://www.sounddomain.com/memberpage/230931/4 . what speakers are you thinking about getting?
There's going to be a bit of modification to the mounts and you'll need a custom mounting plate, but it can be done. I'm putting 6.5 components in the front and already have 5.25 components installed in the rear.
The Q-forms kick panel speaker mounts will give you the best spacial imaging for the money. I'm debating about getting a pair myself and then trying to put a midbass only in the doors (80-200Hz). I did a simlar setup in my Isuzu truck but I actually put an 8" in the door there and it was sweet - it's great that have that much good sounding bass up front. However, the space is very limited in the Focus door unless you wanted to do some outrageously custom door panels.
the kick panels work great, i had them for a while and then i used them for a mold to build some custom ones, mostly just to get some expewirene working with fiberglass, just so you know though, to put 6.5" speakers in there, 90% of them won't fit as a drop in, you'll have to mount them from the back if you know what i mean, you cut a hole and then place the speaker on the backside of the kickpanel and then put screws though the backside and put some nuts on the other side to hold it there. by doign this though, some speakers will cause the kickpanel to not fit flush aginst the carpet, i didn't have any problems with the drivers side kick but the passenger side did that, although it wasen't noticable, just thought i'd mention it.