Your adult piggies are on the small side (800g is usually the lower end of the normal range); it depends on their background. Runts are more frequent in later pregnancies with a larger number of pups, especially when sows have been overbred. Thankfully the good news is that even very small piggies can have a happy normal life, so I wouldn't worry as long as are eating normally.
I would however strongly recommend to weigh all your piggies once weekly for control. The one big disadvantage is that smaller guinea pigs have less extra weight to lose when they are ill, so you want to catch things as early as possible. Weighing is an important tool in health monitoring.
https://www.theguineapigforum.co.uk...deal-weight-overweight-and-underweight.38805/
https://www.theguineapigforum.co.uk/threads/recommendations-for-a-balanced-general-guinea-pig-diet.116460/
You will have to factor in a full 6 weeks post op wait after neutering until a boar is 100%. He can live next door to his future wives with interaction through the bars; that will make for better acceptance. You will find that he will come out of himself in the company of sows. Although my timid Terfel still prefers to hide behind his blind wives in a crisis!
Please try to find a vet with as much practice in guinea pig neutering and small furry operations as possible in order to minimise the risks of post op complications. We can give you tips on how to prepare for the operation and post op care, as well as bonding tips.