Hi there,
Firstly guinea pigs are not very good outside, in mine and many guinea pig experts opinion they are indoor animals. You need to be careful where you research cavies as well since a lot of information in books, the web etc can be misguided, wrong or just plain dangerous.
http://www.guineapigcages.com/
The above website will be able to give you a good insight on what is needed for guinea pig (cavy) accommodation. Since you are getting two 10.5 sq ft is really the size of cage/hutch you should be aiming for. Of course if they are both male then it is a matter of the bigger the better.
Regarding the garage, I personally would keep them well away from cars. The reason they really don’t do so well outside is because cavies are so sensitive to changes in temperature, drafts, heat, and the air around them. It is ridiculously easy how they can get URI’s (upper respiratory infections) – which is extremely serious and more often than not, fatal. Another reason I would not keep cavies outside is because when they do get ill, time is everything, and its less likely you will notice an illness if they are out of mind outside. Not to mention predators likes foxes, ferrets, cats…humans.
As already said a constant supply of hay is vital for their gut and teeth. Guinea pigs need to constantly have something in their stomachs processing. They are foragers and will usually eat a lot throughout the day. Hay is also their favourite toy.
As for hard feed, they need about half a cup full per pig per day, although this may need to be more if they are being kept outside. Only use a pellet, not a mixed based feed. The three best on the market are Oxbowe Cavy Cuisine, Supa Excell Guinea and Pets at Home Nuggets. The mixes encourage fussy eating and also have lots of extra bits and pieces that look good but it would be like feeding children McDonalds every night. Not to mention some even contain such ingredients as peas, colourings and so forth.
Fresh veg should also be given daily – ideally spread over 2-3 meals a day however 1 is fine.
Please refer to this diet chart to understand what can be fed how often a week.
http://www.guineapigcages.com/forum/...ants-list.html
My guinea pigs are given the following each day [per pig per day]…
-1-2 Round Lettuce Leaves (any lettuce apart from Iceburg is okay, although Romaine contains more sugar than Round)
-20-30 Coriander Storks (with leaves)
-¼ of a Green Pepper (contains less sugar than red & yellow – this is such a great source of vit c but NO SEEDS)
-1 hand-gutted baby tomato (seeds removed)
-EITHER 1 baby carrot/1/2 celery stick (cut into small chunks to avoid choking)/1 inch of cucumber
-Every other day one of the following…4xDill storks or 4xParsley stalks
Once a week you may give fruit however anymore often isn’t good for them and not too much. I tend to keep to grapes (1 p/p) and apple (1 inch p/p).
When it comes to Vitimin C, this is very important however a good pellet food along with a good vegetable based diet will be sufficient and do a good job of this. Never add supplements, vitamin drops to water, mineral or salt wheels etc. They are commercial junk and do more harm than good.
As for the petshop thing, I would honestly go to a rescue. It is so shocking how many times I have heard people getting pregnant sows from pet shops (even Pets at Home), or a girl and boy when it was suppose to be two girls etc and ending up overrun with babies. Not only that but the situation with guinea pigs needing homes is pretty bad, and so is the breeding scene behind most petshops.