Sorry for not getting back on this one. "Real life" in the way
Seriously, I would keep it simple. For a mixer get something 8 in, main out plus four aux, analogue. That will give you more than enough for the time being.
Mic up the singers and the guitarist cab only, use one channel of aux for the singers, and initially just one for the rock band. Remember most active monitors will let you daisy chain, so you can feed the same signal to a monitor for the rhythm section and for git/keys. There are a nice pair of Laney monitors for sale on this site ad a good price.
Just work with that for now, and get the FOH sound ok. If the big band can't now hear the guitar because he is turned down, or want a bit more of the band, then you have two mixes available to give them.
You should get a decent monitor and PA mix with that, and plenty of channels spare to DI the bass or keys and fly mics over the big band if you want to.
cheers,
Rich