Most guineas don't need a vet TBH (and sadly, alot of vets don't have that much interest in guinea pigs)
Weigh them, keep a note (or like DH set up, a spreadsheet)
Keep them protected from extremes of temperature and damp and draughts
If they go out on the grass, make sure it's dry enough, limit their grazing to begin with and avoid poisonous plants (I put things over anything like daisies and avoid buttercups, but when they graze they tend to avoid them )
Keep a check on their teeth (you'll only see the front ones). GP1 has broken a tooth off a couple of times but they grow back.
Check their bodies for lumps - can be a fatty nodule which is common, Or abscess , bite, foreign body.
We took ours to a Rodentologist before they went on holiday. They had a worming treatment (Panacur) but I do the Xeno 450 drops on them.(Ivomectin for mites and wormer)
The Rescue you got them from might offer a check up, I know the Rescue I got GP3 does .
It's a good idea to have a search of all the local vets (if you look at the staff profile you should find out if any have a special interest in Guinea-Pigs) .
Then if you do ever need a vet, you'll know where to go.
You could take them for a check to register their details.
The other advice, get a simple Guinea First Aid Kit. (There's a thread way back about this )
Critical Care (Fine Grind) syringes, non fluffy sterile gauze and salt for cleaning eyes, fine tweezers (I had to pull grass out of GP1 teeth when he was gagging) small scissors, Vit C tablets/drops, baby wipes.
GP3 has been with us nearly a year now, from a weeny, skinny little scrap of fur to a big, blustery boar.
GP1 is still our bolshy boar but he's mellowed and lovely .