hey man, welcome to HR.
firstly, a couple of questions. What/how do you intend to use the outboard stuff? i.e are you using it just to record/track with, or are you planning to run stuff back out of Logic 7 into the outboard?
Secondly, i'm not up on my outboard synth stuff but i'm confused as to what you mean by "putting a strain on the mac".
The charm with the Edirol is that is has 4 ins and 4 outs so for recording it gives you plenty of scope. you could setup the focusrite so the signal went: Mic ----> focusrite -----> edirol input
if you wanted the midiverb to add reverb to stuff recorded in logic, you could plug output 3 and 4 from the edirol into the midiverb, set up a bus in logic with the output being 3 and 4, and then create aux sends on your tracks pointed at the bus and treat it like a normal reverb. to get it back in to logic, plug the output of the midiverb into the edirol, create a new track in logic and record the reverb back in as an audio track.
Do you need a mixer? well, i'd be tempted to say no as it would probably just add an extra layer of noise. in my head, you can easily plug the modules straight in to the Edirol like you would a mic. the inputs on the front of the edirol have a three way combo socket that will accept a mic signal, a line level signal (i.e drum machine, electric keyboard), and an instrument signal (with the High Z switch, so you can plug electric guitar, basses, electro acoustic guitars straight in)
In the world of music there's no right or wrong, however there is "tried and tested" and "experimental"