Hi and welcome
Hay/fresh grass fibre makes the bulk of what a guinea pig eats in a day; they should have it available at all times on unlimited offer. The silica in the grass fibre is what grinds down the crucial back teeth and allows the front teeth to self-sharpen in a balanced dental system; it is also what the digestive system is laid out for to break down in two runs through the gut.
Hay should make over three quarters of what a guinea pig eats in a day. It doesn't matter so much whether you feed timothy, meadow or orchard hay, a mix of the three or offering them in turn or parallel - they are the three commercially available grass hay based varieties. Please use the softer hay ypes for burrowing and sleeping in. It is important enrichment.
Any other speciality hay varieties count as treats. Alfalfa/lucerne hay is a legume and not a grass family hay and too rich and high calcium to be fed other than to highly pregnant sows and badly neglected/underweight piggies.
More in-depth information on hay:
A Comprehensive Hay Guide for Guinea Pigs (incl. providers in several countries)
Any other food groups all together replace the supplementary role that wild forage used to have in the diet guinea pigs have evolved on as a species; these include: veg, herbs and fresh forage (1 cup per day) - pellets or dry forage (1 tablespoon per day) - any extra (please healthy enrichment) treats that need come out of the same food allowance; treats include fresh fruit, carrots, sweetcorn, enriched hay etc. We recommend to sprinkle feed any of these supplementary foods around the cage to encourage natural foraging behaviour and reduce food bullying and spoilage. They should not be on offer at all times but they can be used for making friends or as rewards as long as they come out of the overall supplementary food amount.
Our diet guide looks at all the different food groups in heir overall context but then at each group in very practical detail:
Long Term Balanced General And Special Needs Guinea Pig Diets
As to weight: please stay off anything 'average' - what is outside of the big huddle is not automatically wrong; the same as neither any human at nearly 7 ft or somebody under 5 ft is 'wrong'. The natural range of body types and sizes is much wider than any literature suggest and many vet assume. Weight charts are mind traps that should be avoided at all cost.
What you are really looking for is whether your piggy is a good weight/size ratio in themselves or not (the BMI or 'heft'). Being in an personally healthy 'ideal' band is ultimately much more important and life prolonging than being overfed or starved in order to fit an arbitrary 'average' weight range.
These links here explain how weight works and where to feel whether your piggies are in the ideal weight range for themselves at any age (BMI/heft) as well as how to use this for life-long weight/health monitoring; even more so during illness.
Weight - Monitoring and Management
Weight Loss Explained: BMI, Weighing, Poos and Feeding Support
All the links in this post are part of our very practical New Owners information, into which quite literally tens of thousands of questions in coming up to 20 years on this forum and our collective experiences have gone into. They specifically address the basics but also all the most commonly encountered stumbling blocks that new owners come up against. We have tried to make the guides as practical and precise as possible so they are easy to follow.
You may want to bookmark the link, browse, read and re-read at need since you will pick up on different things at different levels of experience:
Getting Started - Essential Information for New Owners
