I assume it's a 4-channel amp that you have then and a head unit with only 1 RCA preout for your phono cables?
It will be fine to run the sub and the front speakers from a 4 channel amp but from my own experience, it won't handle the low frequency sound as well as Mono Amplifier, which only runs on low frequency and 1 channel. By all means, it won't sound crap at all but I am a perfectionist and would want to get the best from it 1st time round.
If it was me and I had 2 RCA inputs, i'd go for one 2/4 channel amp and situate that under the front seat, from there connect and run the speakers from it. I'd then have a mono amp in the boot sitting with the subwoofer. When I get around to doing mine, i'll be installing a 2 channel amp for the front speakers, i'd go for a 4 channel if the Alpine amp I have wasn't so damn good or had alot of back seat passengers.