i'd go with a component set for the front leaving the rear speakers off the hu. i'd still replace the rear speakers though. swap out the alpine 4 channel for their 2 channel. maybe get the pdx mono for the sub also, that way it'll look nicer stacked with the matching 2 channel. and if going for sq, get the type x instead of the r.
and just wondering, but if wanting to run all alpine, why choose a clarion hu? nothing wrong with it, i love clarion hus', but alpine makes nice ones as well, and would make it an all alpine system.