Two things to keep in mind:
1. Your redfoot is not lonely with its current solitary living situation. I have 4 because I enjoy the personalities, colors, etc.
2. The more tortoises, the more space you need. Can you provide ample space for additional adult tortoises in the future? Some may not get along and will have to be housed separately - this requires a good amount of space. Good information on enclosure size considerations here: http://tortoiselibrary.com/ - on sidebar see Housing --> Enclosures.
I have two that were just under a year old housed with two new hatchlings that were ~1 month old. After a couple months I had to remove one of the younger tortoises because s/he wasn't doing well. The other little one is doing fine with the larger two. So long as you provide ample space, I don't think it will be an issue, but you have to always plan for the worst. This means you need a separate enclosure that is ready to go should the new tortoise not do well with the older one.
Quarantine is a process by which you try to eliminate or minimize the risk of introduction of specific viruses, bacteria, or parasites into an existing colony of animals. In the case of our domestic pets, quarantine is used before new animals are introduced or when animals have been away for breeding or showing. When done properly, the virus, bacteria, or parasite should remain with the quarantined/ infected animals and not be spread into the resident colony. Some suggest quarantining for 3 months to 6 months with tortoises.
Basically, keep them separate and don't let anything come in contact with one that has come in contact with the other. Washing hands, keeping food/substrate separate, etc. Different rooms would be ideal.
No one has yet addressed the size issue. Depending upon how big the yearling is, it may be too big to house with a hatchling. The smaller one might get injured.
Well tommy is only 3 inches and the baby will probably be 2 1/2 inches. Or both will be 1/2 larger so not much of a size difference. Well I won't know until I get another one. Thanks!