You can used them as your fronts - they will definitely play louder than the Infinitys - although I don't know that you'll find them inherently 'better' because of that. They will be more 'impressive' if that's the sound you're looking for (some would describe this as 'forward sounding' or 'bright'). One thing I would suggest if you are considering them for the fronts is that you also pick up the matching center from him at the same time. Otherwise it will be very apparent that the Infinitys are designed from a different perspective on sound reproduction. Not to metion the difference in sensitivities will require significant tweaking in the AVR to come close - but even then the voicing will be way off.
As far as having the surrounds be from a different brand - I don't think that matters much at all. Some purists want only 5 identical speakers in order for everything to be perfect - which I don't deny is the optimal setup. However, I don't think the surrounds matter all that much unless you listen to SACD's of certain orchestral performances, and a few movies with 360 degree pans... and even then I'd say 'meh' and go back to enjoying the sound coming from the front 3.
EDIT: Let me add a little clarification to that last paragraph... although I don't think having voice-matched surrounds is very important - I think if you are simply considering the Klipsch as surrounds, then you would probably be far better served by looking for some cheap Infinity bookshelf models that are similarly voiced to your fronts. I made that statement largely for the case of already having a pair of speakers from another brand to use as surrounds or if you can only afford to upgrade your L/C/R speakers without being able to upgrade the surrounds at the same time - thus making your current L/R Inifity's the "free" speaker surround option.
