Dear Cracklespike,
If you are new to Metformin you might be one of those who experience a few of the common side effects, notably an upset tummy for a few days until you become accustomed to this medication. Also to be effective Metformin should also be taken with food.
As for taking Steroid medication as well, this is generally a problem, not so much with the Metformin but with the Steroid medications which are generally known to raise Blood Glucose control levels. I would suggest to you that you seek advice from your Dr over this issue and maybe he/she would then decide that the best course of your treatment would be to increase the dosage of your current Steroid Medication and only take the revised dosage on alternative days so as to minimise the effect it would have to your B/G levels over what would have been the two day period.
Obviously I am not qualified to tell you what to do, so see your Dr and he/she might then decide that this is the best compromise for the time being. Sorry I can't suggest anything else. Good Luck.