OK. I understand your thinking, but it is a little bit scewed. First of all, Shetlands come in many different types across the world. What type is yours? You should know that, having set yourself up as a breeder.
Is your mare Modern or Classic? As I hope you know, the Classic is further broken down into Foundation. All are different. Very different. Now you add some kind of rescue pony into the mix. Not a very good idea but the deed is done. What kind of pony is the rescue? Is he a Shetland or some type thereof?
For what it's worth, 'teams' are usually horses of an identical height and more often than not, identical colours. If you don't know the true background of these two horses, the height and even colours, could be very different and not able to be used as a team. Colour differences would be ok I suppose, but height, probably not.
I am not a proponent of breeding rescue horses of unknown background anyway. Irresponsible breedings, are just why we have thousands of ponies and horses going to slaughter each month, in the most dreadful circumstances.
I know you say you will keep the baby for ever, but things happen. Life changes. Just look at the enormous numbers of horses given away, starving to death and sent to auction. Many of these, were bred by people who thought they would keep that baby for ever.
I know it's too late for your mare, and do truly hope the baby will have a forever home with you, but the likelihood (as with millions of others) is unfortunately slim.
Regardless, I hope you have an uneventful birth and the baby is all you had hoped for. Have the sire and dam been DNA'd as to colour? This will give you a better idea, as to what they might produce.