Running the 2 in series as I explained pretty much turns 2 subs into a single with a higher omb to make it easier to run on the amp.
some newer amps allow you to run a bridge and regular at same time, so it is possible you could bridge your sub and connect the others to the amp also, however, I dont think you need that much power to those other speakers unless you really want to blow yourself out of the van. Lol.
The deck should have a power rating, if it is like 15 watt front channel and 25-35 rear, that is more than enough for 6 1/2" or 5 1/4" or below. If they are 6" x 9" I might consider getting a second smaller amp or equalizer to give it a bit of extra power.
This is all based on opinion and installing stereos in my cars and friend's cars for 20+ years and I am not a pro but as least you get a non bias opinion on it.