Welcome to the forum
Poor piggies. They will improve now they are getting good care
Have they been checked by a vet?
Don’t bathe before seeing a vet. Bathing or using any products can hide physical markers a vet needs to make a diagnosis of a problem so only bathe after the vet has seen them and only if they say it is necessary.
It’s not normal for the coat to be dull and dry.
A dull, dry coat or stiff coat can be a sign of a variety of health issues from malnutrition and dehydration to parasites, fungal or skin issues so all those need to be ruled out if they haven’t already been.
Bathing and using products can be quite bad for their skin. It can strip their natural barrier so bathing can actually make things worse in some cases, therefore so it should only be done if absolutely essential (ie if the coat is dirty, urine stained, or if required as part of a course of skin treatment such as at the end of a treatment for ringworm).
You will see I have added links to our guides throughout my reply which give further information on each aspect:
Please do not put vit c in the water. Nothing should ever be added to the water.
Doing so can change the taste of the water, stop them drinking entirely, promote algae growth. It also means you have no control over any dosages and can’t know that they are actually getting any of the dosage you intend.
All About Drinking, Bottles and Dehydration
Please only supplement with vit c if a vet diagnoses a piggy with a need for additional vit c. Often getting them onto a good diet is all that is needed.
If additional vitamin c is needed (ie if there is a diagnosed case of scurvy), it should be administered directly to the piggy (so you can control the amount and dosage) and only done as a short course (usually just two weeks) alongside getting them onto a good diet.
Generally healthy piggies do not and should not be supplemented with any vitamin c. They get all they need from a balanced diet.
Prolonged use of vit c can end up being harmful as their body becomes used to abnormally and unnecessarily high amounts so if that level then drops, it can cause them to get scurvy purely from the drop. Plus excess vit c is excreted in the urine so if they get all they need from their diets then any supplementation on top is just coming straight out. (Any piggy who has been long term supplemented needs to be weaned off very slowly)
The guide below has a section at the end regarding vitamin c
Probiotics & Live Gut Microbiome Transfer ('Poo Soup'); Recovery Formula Foods And Vitamin C: Overview With Product Links and Transfer Recipe
Make sure to weigh them regularly so you be sure their hay intake is sufficient to keep them stable.
Weight and Weight Loss Explained: BMI, Weighing, Poos and Feeding Support Levels
Introduce any veggies very slowly if they are not used to them as otherwise it can cause digestive problems
How to Introduce New or Reintroduce Regular Fresh and Dry Food Safely - The Rules
Long Term Balanced General And Special Needs Guinea Pig Diets
Keep us posted on how they go
Welcome to the forum. I would say just focus on a normal balanced diet, as you are doing. It will take time for them to adjust, but gradual is the way to do it. Their gut biome needs time to adjust to a new diet of unlimited hay and gentle introduction of veggies and small amounts of pellets. I don't think supplements are necessary if they can eat some veggies, but introduce veggies gradually so they have time to manufacture the tummy bacteria they need. Some very fresh poop soup from a healthy piggy would be useful if it's available. Otherwise, just normal feeding will produce good results. Thank you for rescuing them 
