Bookshelf Front with Satellite Surround?

No, it'll actually sound better with bigger speakers up front. Are you replacing the center too? You should try to match whatever you have up front.

Surround speakers don't get that much info anyway. Mainly background noise. Keep the satellites as surrounds. Maybe even use the other two as surround backs if you have a 7.1 receiver.

What brands are we talking about?

I agree with afrogt, keep the sats as surrounds. And also think it's imperative you match the front three speakers for a timbre-matched, seamless soundstage. If you are ordering the front L and R bookshelves (Energy?) then you'd be wise to get the matching Energy center (perhaps the CC-100). It can be ok for the surrounds to be different than the fronts, but not the center different than the fronts.

The center speaker CC-100 is here for $119. Better jump on it now. One of those woot sales. could be gone shortly.

http://shopping.yahoo.com/?name=woot#woot

If you want some smaller matching satellites to replace the Apex, get these. Perfect match for the C-100 fronts. 8.75" tall. I hope that's small enough.
http://www.audioadvisor.com/prodinfo.asp?number=ENC50

Matching the front soundstage is the most important part of the setup you Will be able to tell the difference, audiophile or not. I would return that polk and get a matching one.

Does your AVR allow you to set a different crossover for each set of speakers -fronts, center, and surrounds? If so, you should utilize this capability to adjust the crossover setting appropriately for the fronts vs. the satellites.

If not, what global crossover setting have you been using with the satellites?

Do you know what your satellites -3dB point is? If they are really small, 120Hz may not have even been high enough for them.

Depending upon their -3dB point, a lower crossover point will probably allow you to get more out of your speakers as well as sisgnificantly reduce subwoofer localization. Quite often speakers can reproduce the midbass in that 80-120Hz region better than the subwoofer can. But, if you lower it to 80Hz, it could be too low for your other speakers and create a gap in the frequency response in those channels between 80Hz and their low-end capability.

So, you may have decision to make. You can keep the 120Hz setting in order to reduce or prevent a gap in the satellites' response, or you can lower the setting to 80Hz if that is more appropriate for your new fronts and center. Or you can compromise, perhaps, and use a 100Hz setting. Because the surround info is often considered to be less important than the front and center info, many people are willing to accept that gap in the surround channels' frequency response in order to run a lower and more appropriate crossover setting for their front and center channels. If the current 120Hz setting is too low for your satellites, then you have a gap already and I would be tempted, even though it will widen the gap a bit, to just use a crossover setting that is appropriate for your new fronts and center.