Need some experienced advice about speakers

[Nexus1155]

So, the truck I have currently is an Envoy with the rear speakers blown out, and the radio on its way out as well. My dad would like to keep everything factory in the truck per say such as headunit and no big boxes in the back so i bought an amp and some midbass's for the rear and have yet to get the front components yet.

My question. I got this amp because it had an 8 wire speaker input so I would not lose any sound from power loss or fading like I did in one that only accepted rear wires and only had power when i faded to rear.

Can I hook up this amplifier to the stock system and speakers for now until I put the newer speakers and components in at a later time, or will the amp overpower the stock speakers and cause problems?

If so, can I install the midbass's in the back until I get components for the front at a later date and just keep stock speakers up front for now?

If anyone is knowledgeable on this subject that would be awesome. Sorry I don't have the model #'s on the products or specs atm they're at home, but the amp obviously puts out more power the stock speakers are rated for, so its a question of theory