When to take RCD prospects to woods.

I cringe at the thought of running catch dogs on the ground with anything other than other catch dogs. (My disclaimer)
Having said that, I'll answere your question best I can for a semi short answere.
A lot of when a pup would be ready to start catching pigs as the main dogs used will greatly depend on the breed being used and crosses there of. A pit and a lot of bull bloods will take some punishment from a pig and not be set back. Often times it will make them harder as long as the punishment they are eating isn't to bad. Conversely, other types of catch dogs such as curs that catch, sight hound breeds, and more will be set back if they take to much heat before they are mentally ready. Smaller breeds tend to mature a lot quicker than larger breeds on average and kinda as a general rule. Size has its advantages when dogs will be holding for a little while but size by itself does not make a good holding dog.
I'll run a 9 or 10 month old catch dog with an experienced dog but I won't let an inexperienced pup hunt it's own pigs. ( I hunt with catch dogs not bay dogs) I start young dogs off by sending them to pigs I can see and won't send them to big groups where they will likely get rallied on, or big loan boars, or a group with a big boar in it because the experienced dog will often target the boar. I want the pup to run 1 on 1 with an experienced dog and get used to holding decent pigs before I send it to the big ones with the experienced dog. I also want my young dogs to be a year old there abouts before sending them to big pigs with another dog. Makes me feel better about their teeth. I don't work a young dog hard either. After the dog has been on several good pigs and is catching pigs on its own when sending it with the other dog, I'll then start hunting the dog as a finder holder. Hunting dogs often push themselves pretty hard and a big breed young dog doesn't need to push itself hard often, or you will be setting your pup up for physical ailments later on in its life. Takes a while for the growrh plates to close and the skelatal system to mature in bigger dogs but same principle should be applied in all dogs. My dogs don't have much if any bull blood in them and taking a little punishment to early one will set some of them back. It's way faster to take it slow with my type of dogs even the ones that have the confedence to go head to head with a big boar, than it is to let that young dog bite off more than its ready for and get set back.
Sounds like you are gonna run these two pups on the ground with bay dogs. If so I will asume they are gonna be the main holding dogs be it till you get there with a lead in bull dog or just let them two keep holding. I don't see why you would need a lead in if you already have two catch dogs holding but people have their reasons. Regardless, those two catch dogs will be taking the initial hook up, doing the brunt of the holding for the longer time and do to the bay dogs, the pig will be caught in a much better place for it to defend itself again the dogs and will not settle where the catch dogs can relax and just hold. The pig will be constantly fighting do to the harassment of the bay dogs keeping the caught hog from settling down thus resulting in much more energy used by the catch dogs to keep controle of the pig thus resulting in them getting hotter than they would if there were no other dogs there at the catch and also do to the pig not settling, the catch dogs will likely be taking more punishment. These dogs no matter what breed will start off with many strikes against them. Not saying they won't make it, just gonna be a rough and rocky road most likely. To go down that road with young inexperienced catch dogs especially those that get set back when having eat some punishment will most likely be the long way around for them.

Hope this all makes sense. Either way you choose to go, just knowing what your up against certainly helps midagate some of the issues waiting.