I stand corrected, then
The 10" drivers in front -- tough to call them mid-basses at that size -- are a rare size indeed for front speakers!
The 10" & 3.5" (and a crossover somewhere in there) together are what make up the 2-way component system. The 5.25" rears must be full-range speakers.
How fares the budget? If you can't up it right now, I'd suggest getting a 4-ch amp for the full $400. You can use it with the Rainbows for a while (in 2-channel config) and then get the Sub a bit later, and run the amp in front + sub style (front channels feeding the Rainbows, rear channels feeding the Sub).
BTW, you'll need to spend a bit of money on power wiring and RCA cords when you buy the amp. Should run to $100 or so.