Well, the park opens at 8:30 so 10 is fine :) Besides albeit is no longer "hot", early hours to walk in a park with lots of uphill ramps is always better than towards noon, when it's much warmer.
As per guided-tours, I can't really help because, in general, I prefer DIY my visits. Yet a good read about the places you're about to visit is always advisable so as to "understand" what you're about to see. There are lots of sources, from guidebooks to Wikipedia to the attractions' sites themselves, many of which contain valuable information (ie. Sagrada Família). Obviously, a guide person can/should be able to provide extra information and details that might make your visit even more enjoyable, but again, at an extra price of course --as in many other industry sectors, the rule prevails: anything involving humans dramatically increases the cost, hence the final price. However, if I wanted to take a tour guide, it probably wouldn't be in Park Güell as, to me, rather than background details, it is more about feeling and experiencing nature in the middle of the city in combination with architecture that mimics nature itself. In short: sensations over knowledge and data
If considering visiting only the monumental area, the visit could be done in say 1h-1h:30' or so, but if including (and it's worthwhile!) visiting the Viaduct http://brbun01.files.wordpress.com/2012/07/182.jpg) located to the right when entering thru the east gate or the Turó de les Tres Creus above mentioned (http://static.panoramio.com/photos/large/49299751.jpg) at the top of the park and with an impressive view of the city: http://farm6.staticflickr.com/5450/8811907184_4a4bcffe44_o.jpg you might want to add at the very least 45'. Both are located elsewhere in the park outside the so-called monumental area and many visitors miss it because they are only focused in that.
Also, the Gaudí Cas-Museum is there, inside the park -albeit run by a different organisation, therefore, you need a different ticket to visit it. And outside the park, at a hundred yards or so, there's something called Gaudí Experiència: http://www.gaudiexperiencia.com/en/
One of the problems in Barcelona is that it's not a cheap city for visitors (or locals for that matter!) and even a short list of sites/attractions to visit, it quickly burns through your budget. Batlló, Pedrera or Sagrada Família are already between 20 and 30€ each, then there's another bunch (ie Bellesguard, Palau Güell, etc) which are in the range of 15-20€, and so on and so forth. Therefore one must be very vigilant and choose wisely :))
I second Ann's suggestion about the exhibit at the top floor of La Pedrera, although not sure if it's still on.